AFAIK, it's not the same cave every game (seems like I've had 3 different caves across my toons who've done this) and I would doubt the helm is there at all if you haven't picked up the quest.... Don't know that for positive of course, but that's how various other things do work.
I know what youre saying but its not 100% that quest items only appear once the quest is triggered. Ive picked up various things like journals, logbooks etc only to find hours later when i do happen to run into the person accidently who activates the quest to retrieve an item, i have the speech option ' i have already found the item '.
I also picked up a silver mold and had the thing in my inventory for what seemed a lifetime. I tried to drop the thing and wasn't able to as it was a quest item. Hmmmm actually i have never tried but can/do you have the option to sell quest items that won't allow you to drop them ??