SureAI

So as i was travelling from Erothin to that burned down village, i came across a castle, killed and looted everyone (including prince). I also checked the vault, but it was planked and render uncontinuable. So i did the quest, travelled through the dungeon. Will there be any continuing to it, or i shouldn't had killed the prince?

No, the quest ends when you find the writings and the armor.

There is a note you find later about a guy in a house in the Dead Pass. He talks about the sacking of Ledur and says something about wishing to meet a survivor. Does this become a quest at any time or is it a dead end so far. I'll admit I haven't found a house in the Dead Pass area, but I haven't searched exhaustively there yet.

It's an easter egg as far as I can tell.

I couldn't picked up "The Old Middlerealm Armor" , but as far as i read in this forum, it's not implemented yet, right?

It's going to be fixed in 1.0.8.2.