1) It depends on what motors a are fitted, if its geared, the resistance in the drivetrain and the propellers you are using.
2) You can use some form of a bottle top, anything from a plasting coke bottle to a jar top.
3) Maplin sell more of the one you already have there and larger covers for the full sized toggle switches too.
4) Not sure, it it a fibreglass hull?
5) Looks like a Type VII
6) No it doesn't need to have reverese if you only want to go forwards.
7) Dont know, but low rev 380's on direct drive should be fine. You can run whatever voltage the motors are desgined for. I try to stick with 12v to keep the current down.
I would stick with 40MHz even for a dynamic diver.