I'm suppose to duel with Bolvyn Venim in the arena pitt, but I never see him or anyone, even though I can hear combat music. Could The Arena mod have anything to do with it?

Yes. There was a thread recently explaining this and there may have been a fix. Hang on for a little while, I'm sure one of the people who were discussing that will pop in.

I'm looking at "Welcome to the Arena" v 2.5 and one problem I can see in the vivecArenaScript is that it uses CellChanged, and Cellchanged can be broken by other mods that attach scripts to doors.

Alrighty. Glad it isn't just me.

Hmm, I don't think I have any mods that's done anything to doors... done again, I have a bunch of mods that I can't keep track of...

john's on it. I believe he was the one discussing it in the other thread. I remember something about "never, never, never" putting a script on a door? Apparently the original MQ is dependent on a script on that door, but the script can be triggered by other duels . . . if I'm not mistaken.

SO... there hasn't been a fix for it yet? Does Welcome to the Arena alter doors?

Yeah, well those braincells are looong gone The thread may be gone too.
The arena is run by the vivecArenaScript which is attached to a ring that is "buried" under the floor of the arena. The "never, never" thing about scripts on doors, I https://sites.google.com/site/johnmoonsugar/Home/morrowind-scripting-tips#cellchanged.

Sogar, if you have tes3cmd and don't mind trying it, the following command may locate any problematic plugins that add scripts to the arena doors:

tes3cmd dump --type door --id Ex_V_cantondoor_01 --match script *.es?

Edit:The problem would be *other* plugins adding scripts to doors, which interferes with the vivecArenaScript using Cellchanged function, as it will never be true if you enter through a scripted door.

Yeah, I downloaded your program from my other thread about cleaning out mods. I'll give it a go.