When it comes to escape velocity, mass isn't the only factor. 11.2 Km/s is Earth's escape velocity at the surface. Its lower the higher you go. This also applies to any object wanting to become a black hole, because if you have something that's a hair away from having the right density to become a black hole (the volume in this case being of the space within the event horizon), its just gonna ever so slightly collapse on itself under its own gravity and become a true black hole.

Yes, but you may run into problems related to tidal heating

Buran was by all means a better shuttle. Far safer, too. It could fly with more payload than the American Shuttle, and it could go all the way from takeoff to orbit to landing on a runway with no crew onboard, meaning that no crew had to be risked for missions that did not require them. It had far more abort options thanks to Energia's liquid boosters, which could be shut down unlike the SRBs on the American Shuttle. If the orbiter wasnt required, Energia could even fly on its own with massive payload making it more flexible.

Unfortunately Buran was really just the product of the cold war tensions. It was developed as a response to some capabilities the USSR thought the American Shuttle had, mainly in the realm of satellite recovery, which it did not. Therefore, and much like the MiG-25/F-15 situation, it was overbuilt. It came at a time where the USSR was speeding head first into economic collapse and despite the spacecraft's (and rocket's) fantastic capabilities, it just was not possible to finance a program like this. Buran ended up flying once, flawlessly, and Energia twice, equally flawlessly (despite the failure in the actual payload, Polyus).

There's not really any reason to put it on a comet instead of just sending it on its own. I guess it could aid in detection but you can also just slap a ton of reflectors on the probe and achieve the same result

If its a truly empty space (a perfect vacuum), sound cannot travel at all. Sound is nothing more than a wave of molecules bouncing into eachother. No molecules, no sound.