regarding Mariliths: I know it was kinda glossed over for being a long quest, but I feel I do have to give a shoutout to Princess Aurelia from Prison of the Planes... because once you are able to activate the side prisons, you can make her respawn as fast as you can DDoor and run back. I was a bit short on beeing able to see marilith HP (want to see it for the new raid) so I farmed her for my last few, and it was surprisingly easy (while overleveled running it on heroic normal)
Make sure you use single target dps or instakills (trap the soul worked beautifully on her). Go in, kill her, DO NOT HURT THE JARILITHS, Dimension Door out. The room only finishes if all enemies are slain, and if no one is in the room, it closes and resets itself. Run back, activate the room, repeat. The limiting factor is waiting for DDoor to come off cooldown to be used again. My DDoor has a 1 minute cooldown, so I was essentially getting 1 kill per minute once I was there.
Randomly it will complete without killing the Jariliths (I think because sometimes they don't despawn with the room reset, but will anyways like a normal summon after 10-ish minutes, so if they despawn naturally at the same time you kill the Princess, if finishes it. I averaged around 40-50 kills before this happened to me while I was farming her for my monster manual, so for me was nothing more than a minor annoyance overall).
edit: it may sound cheesy, but I suspect these rooms were coded like this to prevent soft-locking yourself out of quest completions. Everyone in a room being dead also triggers them to close and reset, and an emergency DDoor to run away from a fight gone bad is also valid.