Yes, I'm pretty sure they just have to be very good friends to get the ask to move in option on the chat menu. Never happened to me, tho. I hardly ever bother to make best friends if they neither are falling in love anyway nor are living together anyway. Takes too damned much micromanagement.
Yes, it seems to work with more or less complete reliability at 100/100. Most things work perfectly at 100/100, and it seems so easy to get to. For me, best friends just kinda happen. If your usual socialize pattern is a holdover from early TS1, before the "lifetime" meter, it seems to follow naturally: Every time you look to repair some relationship decay, the natural stopping point tends to be to push daily to 100. Then you kinda ignore it for bit, and it'll take a few days to backslide down to the point where you need to address the matter. By then, however, since the lifetime meter converges towards the daily meter when left unattended, I'm generally looking at a relationship that looks like 80/60. One phone call about 4 days after the initial meeting nets you a new "best friend". All in all, the social angle is far easier to manage than in TS1. The way the lifetime/daily meters work, this makes it all too easy to repair damage inflicted to a relationship when a romance sim gets busted: Since most of the damage is inflicted upon the "daily" meter, if you address it right away, it's no harder than making a new friend, and if you attack it from cheatee's side, it's even easier, since the other sim will not reject any interactions since he'll still be 100/100 love. On the flipside, if you ignore it, rather than having the anger cool off, it festers, since if you leave a relationship in the negatives, the lifetime meter instead converges towards that negative number.
