Hey guys I had the same problem dawnguard side Dexion attacking in the keep here is what I did to continue:
1. Go around to different alchemy shops until you find an Elixir of Illusion (I recommend having 2 at least, although i used 1). This increases elixir potency by 100% for 60 seconds. Other potions weren't powerful enough for my character who is 75ish. I think the Nightingale Hood adds 20% but regular armor enchants only increase illusion time limit, not potency.
2. Make sure you have the PACIFY illusion spell. It is an expert or Adept level spell. If you do not have it or can't get it yet cast muffle on yourself all the time while you adventure normally and the spell book should be available from the college around 50 illusion level.
3. Enter the keep and just beat dexion down. Once he is down, equip healing hands and pacify. Healing hands to get him up, then drink the elixir of illusion and cast pacify on him (it has a fireball-like radius). He should stand still and all fighting will stop. Begin the next part of the quest by talking to isran or him etc.
4. His story sequence where he reads the scroll is longer than 60 seconds so while he reads it just continue casting pacify on him every few seconds until your potion runs out and the game tells you dexion is too powerful for the spell. After that if you are worried he will attack again drink another elixir and continue to pacify him. Until the quest progresses.
NOTE: After this he will still attack you on entering the keep everytime and hate you forever. You will possibly have to do this again on the next part of the quest chain, but you may just be able to take him down and talk to him while he crawls for that one since there is no special scroll reading animation.
I beat this chain a week ago and I still beat his old @@@ down every time I enter the keep.
Hope this helps you Dovahkiin :)