I was surprised at this too... perhaps you're right that he's not quite with it anymore. My own speculation is a little more sinister, that they may have found some other way to incapacitate him. Wouldn't nerve gas have been ironic?
Interesting. Maybe they did use something to make him docile. I'd be ok with that actually, as long as they didn't keep giving him the stuff now that he's in custody and no longer being displayed.
In principle I'd be OK with it too, except that the whole war was pushed as the way to rid Saddam of his chemical capability... but actually I don't think this scenario is very likely - eg surely less likely than the quasi-dementia/shellshock you mentioned.