SFly, I think you will find that if an aircraft is a day late for a required check that it is illegal to fly it, never mind the other implications involved. Its not the pilot's fault its the company's or the engineer's fault if the check wasn't done. I know the Captain (and FO) is the one on the front line, but there are rules you know.
You might also think that having one of the systems of the aircraft is fine, because there is another one, but again, if the CAA rulebook says the aircraft cannot depart then it cannot depart.
As for smoking on a flight. Well, I think all crew will tell you that they have done the fire and smoke training and have experience of just what a toilet fire is like. I can tell you that I would not want to be flying an aircraft on which there was a toilet fire. Its not so much the smoking as the fact that most passengers dispose of the cigarette in the waste paper bin, which is full of ....paper. Which tends to catch light quite easily. Which creates a big problem.
Now if I said to you, would you like to be in the cruise with a fire on board, you would tell me that is totally unsafe, don't be so ridiculous.
That is why crew get upset about people smoking in the toilets.
It also shows a blatant disregard for the rules and for the instructions of the crew.
Which could lead to other problems.
As for the diversion aspect, I suspect not. Despite your views on pilots most pilots go out of their way to save the company money, arrive on time, etc. and most pilots will at some point fiddle some figures to ensure they are legal for a flight, to avoid a huge delay.
So I suspect that they would not have diverted unless the company wanted them to, or if the passenger got disruptive.
He may have been refused return carriage though.