First off, I'm not sure I post this in the correct thread...well, here goes:

Is it possible (or likely) for townie Sims to move into empty houses on their own accord without NRaas SP installed and with the game's own sp disabled?

It has never happened to me, but a friend of mine claims that it's happening in their game.

Are you guys able to shed some light over this?

I believe it's possible in at least a some cases. The one that springs to mind is if a resident sim is pregnant by a homeless NPC, the game will try to place that NPC in a house somewhere. There may be other scenarios where it happens too, but I can't think of any that I know for sure happen without EA's story progression.

Not sure I've seen the homeless father one but another is when the active household needs a career boss.