Yes
Yes, it is possible for a guppy's tail to be bitten off by another fish in the tank.
There are several possible reasons why your guppy may have disappeared overnight. It could have been eaten by another fish in the tank, jumped out of the tank, or died and decomposed quickly. It's also possible that the guppy is hiding in a secluded spot in the tank.
diseased.
put the guppy fry in a different tank because the adult guppies will eat them
A guppy can stay warm by sitting in the bottom of its tank or by having a water proof heating rock
In all probability at least one of them is trying to mate.
Yes, the fry do quite well in the tank.
Put them in a separate tank.
Well, since the females could be pregnant when you buy them, all male would be better.
A Neon Blue Guppy doesn't really defend its self it usually hides.
I have an Oscar Cichlid in the same tank as mine and I also used to have it with guppy's.
The tadpole may well make a meal of the guppy or at least bit chunks of the guppys fins. I would not advise it.