For 1), I don't think it affects gameplay as you are only going to generate the first 2 encounters from 6 dungeon cards. For me personally, I just put the cards not chosen in the bottom of the pile.
For 2), the blight lord gets an action per player, so one from the blight lord player and one action from each hero player. So in a game with 3 heroes, they would actually get 4 actions per turn if each hero is controlled by a player each.
I watched the demo on YouTube by Silverclutch last night and he specifically called out discarding the dungeon cards not chosen into a facedown pile so it looks like we were right on that one.
The extra action for the blight lord makes sense now that you've explained it. I don't think it could ever be more than one extra action though so it seems like it would have been clearer stating "Blight lord gets 1 extra action per round".
Thanks a bunch for answering!