Re: Belts done - No tensioners!!
I'm assuming that there is no question that the work has actually been done..i.e. new belt etc. Did you pay the £331 it or was that work done as part of the sale?.
If the work has been done as described then strictly he has done exactly what he said he'd do....lambda aside. Whilst I'd be the first to say the tensioner ought to have been changed, its mainly done as a precaution and there is nothing set in stone that says the old one will fail. Its less than ideal though. I'd be more concerned that the tensioning was set correctly.
He is not going to be keen to do further work on the engine, and quite honestly..do you want your car to give further practice to a mechanic (who may be very good) but has little/no Alfa experience. I wouldn't be too keen personally.
Assuming the lambda cures the misfire and you end up with a 2.5 V6 running nicely..which we all hope for..then I would suggest that the only other thing you need is a warranty robust enough to cover failure of the cam belt (i.e. replacement/rebuild of the engine) until the next belt change due..in about 40,000 miles.
There may be other things I've not picked up on, and what I am suggesting is not ideal I fully accept, but I do wonder what you will really get out of the dealer, and whether its just a bad idea to let him near the car again. A good indie can check the tension and probably change the tensioner without too much grief..but that would be your cash of course.
I may be wide of the mark here, in which case accept my apologies, but I hope some of my input is helpful.