It varies between no interview at all, and a full interview exactly like a new hire. Both are done. Interviews are more common these days, because if they're skipped, existing employees may resent the acquihire folks for not having to jump through the same hoops.

This is another reason for the vesting payouts for employees. If there wasn't some reward, and you had to do a full interview, why not just interview somewhere else?

Regarding your last question, it is true that a cabal of disgruntled employees could cause problems for an acquihire. I have seen cases where the acquisition was contingent on a certain number of engineers passing the interviews. Another good reason for a big financial incentive.

I don't think that would lead to better terms in a new deal. The next buyer is likely to learn that the company's employees boned the interviews, which is going to look bad.