Professor Moffat adds, ‘If the multi-billion dollar laboratory experiments now underway succeed in directly detecting dark matter, then I will be happy to see Einsteinian and Newtonian gravity retained. However, if dark matter is not detected and we have to conclude that it does not exist, then Einstein and Newtonian gravity must be modified to fit the extensive amount of astronomical and cosmological data, such as the bullet cluster, that cannot otherwise be explained.’

1) The speed of gravity is also the speed of light, at least as far as General Relativity is concerned. This speed pops right out of the Einstein equations (scientists are testing this right now, though it turns out to be a bit difficult to even write down a theory of gravity where the speed of gravity is different).

2) Black holes can most definitely merge. This is one of the events that people hope to be able to detect with the next generation of gravitational wave detectors, as it is expected to occur frequently as a result of galaxy mergers, where the supermassive black holes at their centers merge shortly afterward. And if it turns out that there is a visible component of the merger (which is possible, from matter that surrounds the two black holes), then we will be able to use these merger events to constrain cosmology. Here’s a video that shows a simulation of such an event:
http://www.youtube.com/watch?v=sG8_uexPhaI

Hopefully we’ll be able to say we’ve detected such a merger within a decade or so.

3) Doubt it.

(1) as the speed of light is a constant, could there possibly be a comparable or analogic constant for gravity?

(2) Is it possibe for two massive black holes to collide and or merge with each other and if so is there a mathetical or theoritical model for this or, indeed, an astronomical or empirical observation which can only be explained by postulatiing such phenomena?

(3) Could these speculations possibly having any bearing on dark mater?

There is the other case, the colliding
galaxies:

Astronomers studying dwarf galaxies
formed from the debris of a collision of larger galaxies found the
dwarfs much more massive than expected, and think the additional
material is “missing mass” that theorists said should not be
present in this kind of dwarf galaxy.

The scientists used the National Science Foundation’s Very Large
Array (VLA) radio telescope to study a galaxy called NGC 5291, 200
million light-years from Earth. This galaxy collided with another
360 million years ago, and the collision shot streams of gas and
stars outward. Later, the dwarf galaxies formed from the ejected
debris.

“Our detailed studies of three ‘recycled’ dwarf galaxies in this
system showed that the dwarfs have twice as much unseen matter as
visible matter. This was surprising, because they were expected to
have very little unseen matter,” said Frederic Bournaud, of the
French astrophysics laboratory AIM of the French CEA and CNRS.
Bournaud and his colleagues announced their discovery in the May
10 online issue of the journal Science.

So, how can the Dark Matter Theory explain this observation ?

As concerns the Bullet cluster, it would be interesting to map
also the distribution of cold hydrogen (before starting a
respectable hype about the First Direct Observation of the Dark
Matter (at the risk to mould a bullet:)); one might be even not
convinced that we observe the end of cluster collision,
not the beginning.