Questions re significance of 2016 and 2020 LIGO observations Phys. Rev. Lett. 116, 061102 (2016) - "Observation of Gravitational Waves from a Binary Black Hole Merger"
(https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102) reports that the gravitational waves detected by LIGO match up with the signal expected from two black holes merging as predicted by general relativity.  Additionally, the masses of both black holes were estimated.
Also the article "Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences" (published in 2020 January issue of the "The Astrophysical Journal Letters") states that "the LIGO–Virgo detector network observed gravitational-wave (GW) signals from two compact binary inspirals that are consistent with neutron star–black hole (NSBH) binaries. These represent the first confident observations to date of NSBH binaries via any observational means". https://iopscience.iop.org/article/10.3847/2041-8213/ac082e
Questions:

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*How certain are we that LIGO 2016 and LIGO/VIRGO 2020 detections of gravitational waves are necessarily from two black holes merging and Neutron Star–Black Hole Coalescences (correspondingly)?


*Since supermassive black holes are typically at the center of a host galaxy, what happened to the galaxies that contained the two merging black holes (when such merging is inferred only by the analysis of the structure of  the registered by LIGO gravitational waves)?
Is there additional (independent of gravitational waves capture analysis) observational evidence confirming that black hole's host galaxies indeed have merged.

Edit
After reading the answers, I understand that some signals (filtered from other captured signals, deemed as the background noise, because those signals do not match characteristics of gravitational wave signatures predicted by Einstein's GR) detected by LIGO are inferred as the evidence of two black holes or two neutron stars or black hole and neutron star merging as predicted by general relativity.
When additionally gravitational waves, caused by the merger of the supermassive black holes (those which ARE indeed located in the centers of the merging galaxies) will be detected, since mergers of galaxies could be observed by the methods independent of gravitational waves such as registering the fact of the quasar appearance (source).
Additional questions:


*Could someone offer me the historical ASTRONOMY precedent when the second degree of inference (from the mathematical model, being associated with the theory of Physics) was accepted as the discovery of the astronomical object?


*Could someone give a reference to trustable scientific publication, with the subject of study being discussion of requirements satisfying claim of astronomical discovery of an astronomical object?


*Have independent scientific sources (outside of members of the LIGO team) analyzed methodology and results of the LIGO signal detection, and published their conclusions with regards of what was actually discovered?
PS
 A: levitopher's answer clears up a bit of your confusion regarding the type of black hole involved. The two black holes thought to have produced the detected gravitational waves were ~36 and ~29 solar masses in mass, nowhere near the mass of typical supermassive black holes. They are instead relatively massive stellar-mass black holes.
Regarding your other question - it is indeed quite likely that the source is in fact a pair of merging black holes. Regrettably, no other gravitational wave detectors were online at the time of detection by LIGO. Additionally, the Swift gamma-ray detector reported that it found no counterpart to the merger in the small part of the electromagnetic spectrum it surveyed (Evans et al. (2016)) two days after the gravitational wave detection, and the ANTARES and Ice Cube neutrino detectors detected very few neutrinos at the time of gravitational wave detection (here). However, these results are not disappointing, as Swift's results came later, and the neutrino detections helped put a "concrete limit on neutrino emission from this GW source type", according to the paper.
However, as Kyle Kanos pointed out, the Fermi Gamma-ray Bust Monitor detected a short source of photons which may be connected to GW150914.
The LIGO results present very strong evidence that the event is what it is predicted to be - the merger of two stellar-mass black holes. It passes the 5-sigma threshold confidence level. See here for a short explanation of the team's conclusions as to the nature of the event.
It is also worth mentioning that analysis shows that the observations match all predictions from general relativity.
A: I think it is highly relevant that in 2005  the numerical simulation of more than one orbit of a binary black hole merger was successfully achieved by several groups (examples: 1; 2).  These papers solved a problem after 30 years of concerted effort and moreover they presented the expected expected signal that a gravitational wave interferometer should see given such an event.  That is the basis of the interpretation of the chirp signal that was seen.  
So, this was very much an "if Einstein's theory is correct and if you detect a black hole merger, this is what it will look like" moment: in other words a prediction of a possible outcome of the LIGO experiment.  If you like the Popperian approach to science (of which I am not a strong supporter) you could say that Einstein's theory made a precise prediction that was later verified.
My conclusion is that, taken together, the numerical experiments show that Einstein's theory looks better all the time, numerical simulation has advanced in leaps and bounds, and the LIGO experiment is a triumph of human engineering and scientific foresight.  I personally don't think "truth" has anything to do with it.
A: I'd like to point out that the recent discovery by LIGO was specifically one of gravitational waves. The claim that the cause of the waves was a merger of two stellar kerr black holes is more inferential than by virtue of direct detection. The precise form of the chirp and the ringdown definitively show that the involved objects are compact masses (instead of say, neutron stars, which would have different frequency modulation even prior to the collision). As the signal also includes parts of the orbit where the gravity is relatively weak (where post-newtonian approximations still suffice), one can still distinguish compact masses from fluid spheres even without invoking all of GR in complete detail (the fact that any alternate theory of gravity must still reduce to relativistically corrected newtonian gravity, implies this is very strong evidence of the objects involved in the merger being compact).
As for whether the two colliding objects are black holes, that in my opinion is still an open question. Conclusively detecting a black hole would involve pinning down the event horizon, which at the level of the present detection sensitivity is not possible. The quasi-normal modes post the merger do hold crucial information on the existence of an event horizon. The fact that we see exponentially decaying oscillations at the end is by far the greatest evidence for the final object being a black hole. The issue here is that to conclusively verify this, one needs to be able to resolve a bit more of the frequency spectrum of the quasi-normal modes. Extracting this information by fitting the signal to numerical relativity simulations pre-assumes the validity of GR in the strong-field while perturbative analytical calculations become valid in a regime where the signal dies off into the noise. Hopefully with further upgrades, when LIGO begins to function at design sensitivity, we should be able to resolve this better. The current argument put forth is one of ignorance, that we do not know of any astrophysical mechanisms and/or exotic matter that do not lead to a collapse to a black hole (at ~30 solar masses).
As for the validity of the detection itself, I think quite a few people have answered before me about this and hence I shall not say any more on it.
A: Just as an addendum to everything else, LIGO is tuned to observe mergers of stellar-mass black holes.  The frequency of the gravitational waves is determined by the mass of the system, and since SMBH will have much larger masses than stellar mass black holes (the LIGO holes were 30 solar masses, the SMBH in the Milky way is something like ${10}^6$ stellar masses), they will have frequencies much lower than what LIGO is sensitive for.
This, in fact, is the motivation for LISA, which WILL be sensitive to events related to SMBH and to low-frequency cosmological background gravitational waves.
A: The two black holes observed by LIGO were around 30 solar masses each - they were formed from stellar sources - that is, a supernova or similar event. They are not the same "kind" of black holes which are found in the middle of galaxies.
(sidenote: The fact that they are 30 solar masses is actually interesting. In this paper they discuss how the environment had to be a little bit special for these black holes to form).
In regards to the condition of "truth", it conforms to established scientific norms. For instance, the detector has been very well-modeled and every reasonable error has been accounted for, so we have very good reason to believe that the signal is real (to say nothing about the fact that it was observed in TWO detectors, one in Louisiana and one in Washington, and the signals are nearly identical). To determine the details of the merger, people have been working very hard over the past decade to develop a library of signals, for a variety of objects (neutron stars and black holes) and a wide variety of parameters (masses and orbital parameters). So they determined the characteristics of the merger by comparison with those models.
Of course, we aren't in a spaceship floating over this merger viewing it with our own eyes. But on the basis of the scientific method (hypothesis testing and independent verification), this establishes the existence of gravitational waves.
(for the full paper talking about the observation)
EDIT: I'm going to try to tackle your clarifying questions.


*This one is slightly tricky, since all (extra-solar) astronomy is indirect in this way - we only observe the cosmos via the light we receive from it. For example, the existence of the star Polaris is indirect, and depends on the assumption that stars produce light (which is on very solid footing, obviously). Some examples that might be closer to what you're thinking of - Dark matter is only detected via it's gravitational influence (never directly), but most people consider it to be a real phenomena. Pulsars being associated to neutron stars is mostly theoretical - although we can associate them to SNR sometimes. And actually, the vast majority of extrasolar planets are detected indirectly, via the Doppler shift or transit methods.


*I think the answer is "no". You would have to explore each one individually, since the argument in each case is rather unique. I once listened to an interesting podcast about how astronomy is observational, not experimental. I think it's here. I think the best you can do is list evidence for discovery and let the community decide. This is not a unique problem, BTW - no one has ever seen a Higgs particle, in the traditional sense - we inferred it's existence at a level sufficient for the scientific community.


*LIGO releases it's data to the public at proscribed times. Here's a list of projects using LIGO data. I don't think I see specifically what you are interested in ("We checked LIGO, it's right!"), but this list is only the past few months.
