As it stopped when you fitted the seal, I think you were right and exhaust fumes had been getting into the engine bay. With one cylinder not firing it would have been petrolly/exhaust smelling.
He certainly was not, I used to pay Paul to come and weld my van long before I started doing this stuff for a living.
He won't do that until the air leaks have made the engine go all lumpy as the valve seats regress into the head.
Spark powerful enough? Spark at the right time? Enough fuel getting into the cylinders? Compression enough? Valve clearances sufficient? Count yerself lucky it's a T2 problem, any modern vehicle it would be a nightmare