Actually, Rob Morrow's character Don doesn't "live" there, he just hangs out there a lot. Remember the bit from the first episode, when they were talking about the "hot zones," and he said something like, "If you tried to figure my 'hot zone' based on my address it'd come out wrong because I'm hardly ever at my apartment; I'm always at work or here [Dad's house]." So he does have his own place, we just haven't seen it yet, because he's a workaholic. I don't know about Charlie, but the way he's portrayed, maybe his head's just so full of the math stuff he just hasn't bothered to think about moving out. Or maybe he stays for the blackboards in the garage?

As for the accent, I think I'm in agreement with bibliotrope on both counts.