The bonus exp isn't greater then the base exp you get for doing content closer to your level.
I don't think most people care enough about the difference to take a potentially nightmarish Orbonne run over a piss easy quick LotA/ST run. Whatever the difference is, it's nothing like the difference between, say, Castrum and Prae.

Also while the max hp passive Regen debate is going, I usually do fates or gathers while waiting and I see it happening to myself on occasion so its not a guarantee that they are ilvl cheesing.
From what I can tell, entering any DF thing fully heals you (recharges all cooldowns too!), so even if you were missing HP as a result of being swapped to a crafter class or something, you'd still be right as rain after you enter. The missing HP is definitely a result of reequipping gear after you've already entered (which, as has been pointed out, is unnecessary; you are able to put your gear back on right after you enter the queue)