Im doing the vampire side. SOMEHOW a dawnguard and an armored troll got in the barrier, I used mayhem to kill them, as well as everyone else in Forebear's Hideout. However, this turned Dexion permenoantly mayhem'd. I used Vampire's Seduction on him, then bit him to thrall him, then told him to go to castle volkihar. When I get there, everybody tries to kill him every time he gets back up. When in Forebear's Hideout, the quest description said that I had defeated the Moth Priest long before deactivating the barrier. What do I do?

You may not want to hear this, but the best way to fix this (and the only way to fix it if you're on XBox or PS3) is to reload a save prior to entering Forebear's Hideout and replay that quest. Hopefully you made a save right before you entered the cave.

Keep trying until everyone spawns where they're supposed to spawn and you're able to successfully complete the quest. And you'll have to complete it successfully; otherwise you can't get Dexion to read the Elder Scroll and your game will be completely broken. This is a major bug that Bethesda couldn't be bothered to fix for the final release of the game - it's inexcusable that they didn't.

Did you transform into Vampire Lord at anytime in Forebearer's Hideout? That could cause issues with Dexion if you did and I agree with Gator Country, if you have a save before you entered Forebearer's Hideout, you will probably need to reload it. You could also try waiting 10+ days and see if that works but you'll probably still have to reload.

Then I guess I'm using Embrace of Shadows + Muffle to get to Malkus, pickpocketing the Weystone Focus, using Hysteria, seduce/command Dexion, then run like hell

Edit: it worked and dexion read the elder scroll