Hey,
I'll try to answer your questions:
1. After level-up (i. e. when the trade requesting character has a higher level than the character who caused the last level related restock) and after every hour real time since the last restock (restock always requires trade/pickpocket request).
2. No. They never clear inventory. But there's a mod that clears trader inventories, beside a lot of other changes:
http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=589467&nt=2&page=1One of my mods offers two containers that remove their content when sold to NPCs, beside some other functions:
http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=584472#Post584472 Maybe options for a second playthrough of EE. I personally always prefer the original experience for my first playthrough.
3. Yes.
4. I don't think so. I'm not 100% sure (would have to look into the files), but vendors in Cyseal should still sell low level skillbooks.
