I've had every one of my houses empty since about generation 3. They've never refilled, possibly due to it being an older save, but as far as I know, even in newer games the houses stop populating after a couple generations.
True. Although I have had a couple of houses filled by "new" townies, and once even an erstwhile "homeless" got a home. Pollinating doesn't seem to make a difference either, as far as I can tell.
For the friendship bug, has anyone tested out having someone you've met before the patch get invited to your lot and then tried to make them a friend or bff? If this works, it could maybe also work for the social butterfly aspiration for kids if enough children were known in advance.
As for putting those buildings down, I think Marian T has a good point. We've got an entire town we could fill up first and it could even be interesting to do so and make it the place to go for fun and recreation. Once it's filled up then put any other buildings down elsewhere.
EDIT: I just tested out various ways of making friends. If they are someone you DO NOT know, just walking by, and you greet them and start the friendship routines, you can make them good friends. If they are someone you met elsewhere and ask them to hang out on your lot, the relationship never gets past impartial even with the green bar full. I did not try making that person an enemy and then starting a friendship. I used tce and world needs filled and no decay to folks from wandering off to pee, eat or sleep. It took a full Sim day to make a good friend so without using a cheat I can't see where you could make a friend without them leaving or having to ask them to move in or something.