I'm not sure the studies that coorelate longevity with having many close friends explain why... it's just a coorelation.  Need more research (e.g. keep us researchers employed!).  I think you have put forth a few good hypothesis.

I'll put forth another.  Friends tend to inspire each other... to go to the gym... to get out and go for a walk... to finish the remodel so you can rent out the room and stop throwing away that good rent money...