Alright people, I've posted a walkthrough file on Nexus that covers the entire quest. Do try and resist the temptation.

@Dowaco: If he accepted the offer but won't follow, it may very well be that you've hit the companion limit. I think Oblivion will only let you have two. I myself have never tried taking more than two with me anywhere simply because they tend to get in the way.
It may also be something in your load order altered his NPC record which caused the AI packs to be overwritten. Fastest way to check would be to have tes4edit load your full load order, open the branches for BrotherhoodRenewed.esp, check the NPC branch and find his record there and see if anything other than this and Oblivion.esm are touching him.
If something is, you can dodge the issue with Wrye Bash 285 since ABR is tagged with Actors.AIPackages.
(Er, ok, you posted again saying moving it down in order fixed it, definitely fixable via Bash then)
The thing with the barrel being blocked. Forgot about that. It came up in beta testing, that's OOO blocking it. ABR will need to be loading at least after OOO to prevent that.
@Johnn123:
Sounds like I've got a missing stage condition check missing on exiting the Bruma sanc. I had to use some tricky trigger scripting to keep her minions off your back.
@All:
Aside from the OOO issue Dowaco reminded me of, there is also a conflict with Vaults of Cyrodiil and the Chorrol sanctuary. It can't be resolved via load order and patching it is equally impractical. However Vaults of Cyrodiil does not make it impossible to complete the Chorrol quest.
Spoiler You just need to go back around to Crumbling Mine where the hole in the ruined wall is, and it should ask if you want to squeeze through. You'll be on the other side, in VoC's treasure room, with the pile that belongs to ABR. The blood of the Night Mother is accessible there.
It's awkward, but it works.