As for the "about people" stuff... well, I can't say that there's much there either. The Tam brother was on the ship for months on end, with his sister mostly wandering around in a daze, and he "never had time" to hook up with the engineer gal? That's just lazy sitcom plotting.
Did you watch the series? Simon spent most of it putting his foot in his mouth repeatedly when it came to Kaylee. It wasn't a time issue, he was just out of his element. His reaction to her advances was always a bit awkward, which I can completely understand.
It might be a bit non-obvious since movie Simon was a lot more take-charge than series Simon. In the series, Simon only ever really took charge once, in "Ariel", when the plan was playing out in a setting he knew. The rest of the time he seemed very much adrift, and pretty much clung to River as his only link to something familiar.
I'm not going on "real" science here, I'm going on the rules of the franchise, that ships are incredibly difficult to fly and maintain. When he showed total crazies getting the better of perfectly sane people, he broke his own rule.
I never got the impression anything was "incredibly" difficult about maintaining ships. It wasn't something anyone could do, but it obviously didn't require a PhD.
"The parts are crap, but you put them together, you got a Firefly. Thing'll run forever, you have a mechanic who's even half awake. It's a good catch. She comes this way, you prep the nets."
-"Our Miss Reynolds"
On the other hand, "Out of Gas" showed that the entire ship could be disabled by the failure of one part, which shows the design does have some flaws.
"Catalyzer's a nothin' part, captain."
"It's nothin' till you don't got one. Then it appears to be everything."
On the third side, once Mal *got* the replacement part, it seemed to be just a matter of plug-and-play to get it installed.