No. A little confused by the description of a threaded 90 and having to cut the valve free. Every shower I have ever done the final connection to the valve goes on with a nut. You should be able to take one side loose and leave the valve in place. Disassemble to the leak and replace.
Depends on what you are doing. soldered is for copper water lines. Threaded can be for water, gas, or anything else.
no
It is a round threaded disk shaped thing that screws into a tee or other "female" threaded fitting to " plug" it up.
Yes
Threaded On End
Should be a threaded flared fitting
You can't. There is no fitting to do this. Galvanized has to be threaded.
A threaded copper fitting on the copper side, male or female, and the galvanized is screwed into it.
Exactly what it says threads are crossed inside the female fitting
A coupling
Yes they can.
Yes, first the Galvanized pipe has to be threaded with a standard thread for pipes, then you buy the correct female socket threaded plastic fitting, screw it onto the pipe using Thread pipe sealing tape, then glue your piping into the socket part of the plastic fitting, simple. Cheers.