Mrs. Deagle believes that the gremlins are there to take her to Hell, which, in a way, might be exactly what they did.
When Gizmo gets wet, it produces more Mogwai. Unlike Gizmo, the Mogwais are mean and unruly. Assuming the reason why Gizmo maybe the only nice Mogwai is because he was raised to be a good creature. None of the other Mogwais were raised to be nice and cause chaos.
Or, to look at it another way, copies of anything never have the same quality as the original...
However, it's quite likely that the salt content and other contaminants present in sea water make it unpure so as to be unfit for their reproduction. Freshwater lakes, however, including the GIANT ones in North America (which NYC is not that far away from)...
The very first draft of the script ends with Billy luring all the Gremlins to a greenhouse with one he has trapped in a toolbox (in the script, the Gremlins will go to any lengths to regain a lost member of the group). However, waking up in the hospital later, he realizes he never opened the toolbox. Cut to a worker cleaning out the greenhouse, seeing a nice brand-new toolbox, and putting it in his truck, only for him to get scared and toss it away when funny noises start coming from inside. It lands in a lake. . .
An offhand radio broadcast implies that the Marines were going to hose down the gremlins if it weren't for Billy and Gizmo killing them all first.
How does a gender-flipping serum work on an asexual creature?
It doesn't. Look at the Gremlins' faces in the second movie, and I mean REALLY look, and you'll notice there is clear sexual dimorphism in the species. Of course, considering how they reproduce, it would fall under Purely Aesthetic Gender or something.
Even if we assume that the Gremlins are an agender species, a serum which would, in other species, cause Gender Flip could still have *some* effect on them. Apparently, that effect is the one we see, with the exaggerated appearance of some body parts, and the Gremlin in question thought it looked amusingly similar to a human female and ran with it.
You can't feed them after midnight, but he is never told what time in the morning he can start feeding them again - it's *always* after midnight!
Why haven't these creatures taken over the world á la tribbles? Surely some time in the past some wild mogwais would have got caught in the rain...
If Futterman is to be believed, it happened before.
There are no "wild" Mogwai. The old Chinese shopkeeper was lying about Mogwai being one of "nature's gifts". In the novel, we find out that Gizmo fell from space many years ago, and in the game, we find out the whole Mogwai species were created in genetic experiments by aliens to be the perfect pets. The experiments were considered a failure, obviously.
Nope. The novel is non-canon, and the writer didn't even see the movie (at least not at the time the movie was being made).
The sequel actually devotes a scene to people quibbling over the rules. What would happen if a Mogwai was eating on an airplane that changed time zones?
Given that direct light is painful to them, but ambient doesn't hurt them at all (otherwise, they'd behave like Grues), it's possible that what happens is that any amount of daylight at all prevents them from undergoing the necessary metamorphosis, and the "after midnight" rule is really "at the darkest part of the night". Feeding them after dawn is probably safe.
How can the Gremlins drink beer and dig through snow and not multiply?
Not pure water.
The Gremlins read-along books (merchandising tie-in with Hardees, if memory serves) has a bit where Mr. Hanson explains that water has to be "above freezing" for them to multiply. One presumes water past the boiling point (and thus turned to steam/water vapor) would also not work.