As far as the stones go, actively altering an area that small probably isn't that big of an issue from a coding standpoint. Annoying, most likely, but not entirely impossible. I would say that, at most, the immediate terrain within 5-10m may change a bit.

Then again, I could be totally wrong; I guess we'll see.

As for the trader.. I assumed the only "updates" to him were going to be his inventory, which would mean you'd simply have to go speak to him again to see the updated list.

I kinda would like to see the poor guy get a hut or something, though. Maybe offer a quest that awarded you with some kind of mystical object used in crafting a high-tier weapon or armor? Would be neat!