[Lava]

But area where you find hand, you can go another areas down, at bottom I think there is some tree, and it have "container" but it was empty. Is something supposted to be placed there?

You can talk to the tree, there is a dialogue circle under it. You may offer the tree some of your memories and get rewards--for a price. If you do, the items appear in the tree's hole. 

Quest My Very Own Hideout. I get it now, and did also number of other quests from SotsC so far. Seems nice, hope containers store items and dont delete them when you progress game (you could guess my idea already. For same think I was using containers in Durlag Tower in past).

Could also be fine place to keep NPCs for easier party rotation.

Yeah, you can do that, but remember that if you try to abuse the area, you may get some bugs (like for keeping OC companions and then try to join them in SoD) -- so please, just use it for resting and storing items. When it comes to this area, I can only hope players won't abuse it, I cannot do much more and prevent it in a different way that deleting that part of the mod and I don't want to do that.