Is there a model relating GW signal and luminosity of the merger / burst? To understand fully the importance of the recent observation I would like to know if it is possible to infer the distance or luminosity (in whatever wavelength, or say, the overall power) of the source (or those of the burst after merging) of GWs. This could locate the event by two independent measurements thus improve confidence on current knowledge of distance 
A second question, if it makes sense and has an answer that does not require GR maths: how GWs decay (if they do) while travelling, and if there is any shift due to expansion of space. In my naive interpretation there should not be but I apply a very simple reasoning here (just consider "proper points" in space, the passing way should not change its frequency - Assuming  G wave are not single impulse but a frequency can be defined). 
 A: Yes, the distance to a merging binary system can be obtained solely from the gravitational wave signal.
The amplitude of the signal depends on the frequency of the signal (twice the orbital frequency), the chirp mass and is inversely proportional to the luminosity distance to the source.
At the same time, the "chirpiness of the chirp", that is, the rate of change of frequency, also depends on the chirp mass and frequency.
Therefore by measuring the frequency $f$, amplitude and $df/dt$ of the wave, one can eliminate the chirp mass and estimate the luminosity distance.
This is why coalescing binaries are known as standard sirens.
Note that the "early" part of a coalescence GW signal is degenerate between the redshift and chirp mass, because GWs are indeed "stretched" by redshift in the same way as light; but the later coalescence phase provides individual detector-frame masses and these can be corrected to source-frame masses if the redshift can be estimated.
If an optical counterpart can be identified, then a comparison of its redshift and the luminosity distance yields an estimate of the Hubble parameter and tests distance measurements from the cosmic distance ladder (e.g. Abbott et al. 2017).
A: The intrinsic amplitude from an binary merger is set by the total mass $M$ of the system. However, the final observed waveform instead depends on the "redshifted" mass $(1+z)M$ instead. Redshift and total mass, only enter the waveform in this particular combination. (Certainly in the dominant modes of the signal, subdominant modes could break the degeneracy.) Consequently, a heavier more distant binary looks (almost) identical to lighter closer binary. This is the reason that for most GW events the luminosity distance is determined only to a factor of two or so.
However, if you have an independent measurement of $z$ (from say an electromagnetic counterpart), you can infer to total intrinsic mass of the system, and as a result the intrinsic amplitude of the signal. From this you then a measurement of the luminosity distance.
The combination of the luminosity distance and redshift then allow you to determine the expansion rate of the universe, independently from the cosmic distance ladder.
To answer your second question, GWs experience (cosmological) redshift in exactly the same manner as electromagnetic waves do.
