only thing you get from Magnus is the opening of the stairmays
between the leveles - i.s. no new "presrnts" as such since the upper level has already been "restored".You may be correct about the chests not changiong, but the Castle does respawn in two years in the vanilla game. The rates and bats reappaer as well as the goblins . . nothing else changes though. I never had a Moid go that long so I don;t kinow if it will happen in this mod or nor.
You will get the key to the locked chest when you do the Hero Quest for the Angel.