Although the KotN questline is over and Sir Amiel has vaporized, his ring is still a "quest object" and cannot be removed from inventory. Is this normal? Perhaps I always need it to enter the undercroft at the Priory?

Yeah, it's a Quest Item, can never be removed. I guess in-game, it would make sense. It's the only thing allowing you to enter the undercroft, so if you lost it, you'd be really screwed if you needed to get in there.

In my games, Sir Amiel’s Ring looses the quest status fairly early. I just loaded a save made shortly after entering the undercroft the first time (Dec2008) and my character wasn’t wearing the ring, and she had found only a couple of the relics at that point. I found the ring in a drawer at Arborwatch. I went to the Priory without the ring and entered the undercroft with no problem. Now, five years later, my character wears the ring all the time after it was recently enchanted with 20 Shield using a Sigil Stone.

If you are on a PC, old unwanted quest items can be removed with the console.