The level difference is the cause of not taking damage from those opponents. The level effect for combat will be reduced in the next patch / FoV / DKS.
I don't think you can destroy the hell gate... it has been awhile since I went through that part, though.
You can not get into the house beside the well cave entrance and teleporer unless you offer to help Gobie with his ghost problem (one of several ways to complete that quest). In the house beside that, AFAIK the key on the shelf there is for a nearby chest.