Is the speed of propagation of Einstein's gravitational waves unique?
No. As per all waves, the speed varies according to the properties of the medium. Moreover a gravitational wave is a propagating transient change in the properties of the medium, space, such that optical clocks run slower whilst the wave is passing through. Do note that the speed of a wave also depends on the type of wave. For example seismic waves might be longitudinal P-waves or transverse S-waves. The latter propagate at circa 60% of the speed of the former.
The LIGO Scientific Collaboration and Virgo Collaboration have announced that on the 14th of September 2015, at 09:50:45 UTC, they detected a transient Einstein gravitational wave, designated GW150914, produced by two merging black holes forming a single black hole. They stated that the speed of propagation of the gravitational wave is the speed of light. Is the speed of propagation of Einstein's gravitational waves unique or arbitrary?
It varies, as does the speed of light:
None of the answers others have given to this question are correct. The speed of propagation of Einstein's 'gravitational waves' is arbitrary because it is coordinate dependent.
IMHO you should employ the CMB reference frame here. When you do, you will still find that the speed of a gravitational wave varies. It goes slower when it passes through a region of low gravitational potential, such as a galaxy.
There is no unique speed of propagation. Select a certain set of coordinates then the speed is that of light.
We can't be totally confident that the speed of gravitational waves is the speed of light. I know that's what Einstein said, but the proof is in the pudding, and I'm not totally convinced about the LIGO result.
Select a different set of coordinates then the speed is entirely different to that of light. The claim by the LIGO-Virgo Collaboration that the speed of propagation is the speed of light is patently false.
I don't think it's patently false, in that they had two waveforms at two locations, with a delay that was in line with propagation at c.
Furthermore, General Relativity cannot localise its 'gravitational energy'.
It can. The energy density of space where a gravitational field is, is higher than the energy density of free space.
Consequently there are no 'gravitational waves'.
A wave is a propagating field variation, or a field is a standing wave. If you can have a field, you can have a wave.
Indeed, General Relativity violates the usual conservation of energy and momentum for a closed system and is thereby in conflict with a vast array of experiments.
It doesn't violate conservation of energy. The ascending photon doesn't lose any energy. The descending brick doesn't gain any.
As for the mathematical theory of black holes, it violates the rules of pure mathematics. It is therefore false. LIGO did not detect gravitational waves or black holes. All this has been explained in detail in my paper here: Crothers, S.J., A Critical Analysis of LIGO's Recent Detection of Gravitational Waves Caused by Merging Black Holes, Hadronic Journal, Vol. 39, 2016, http://vixra.org/abs/1603.0127
I'll take a look at it. As I've said before, I think you should reserve your accusations for the incorrect interpretation of GR rather than GR itself.