The Bezel ring: yeah, you're right about that. I didn't think about that that way!
Eagle/Death: Around those grates, huh? I'm sure I've never found the Eagle Sword, but I might have found the Death Axe (somehow, it rings a bell now you mentioned the grates).
The helm "Talamar" might have something to do with increasing your magic or something, but I've never noticed any differences.
I don't know much about the niches in the Dungeons. I believe you could find disappeared items somewhere on the floor...
I also noticed that 'wall-shoving'-sound in the north east. I've searched a very long time, with no result, but after I used a trainer to get behind the wall I found... a stair to the Yvel Woods! This part of the wood is located behind Vulture's Chasm and is not fully completed. You can find an Ace of Oblivion and some kind of wand as well. It appears there was supposed to be a shortcut out of the Dungeons, but it didn't make it to the final release.
So my guess is that stepping on that invisible plate opens and closes the wall.
What you're saying about the shields really surprises me. I thought those shields were mere decorations. Do you also know if a creature uses a weapon (like some Orcs do) also do more damage?
Also, when a shield is destroyed, does that creature's appearence change? It would be very strange if I wouldn't have noticed that!