Yes all brass object can be recycled.
Not necessary, but recommended along with pipe thread compound.
Yes, you can either solder them or fit mechanical (threaded) fittings.
Take either an SOS pad or a wire brush and rough up both the inside of the brass fitting and outside of copper pipe, put the copper pipe into the fitting and solder around it.
If you do connect copper pipe to galvanized pipe, you will need a dielectric coupling or else the galvanized pipe will corrode. Same with connecting galvanized pipe to brass fittings. Brass is an alloy that contains copper and therefore the same electrolytic properties will exist unless a dielectric coupling is used. Hope that helps you out.
Any pipe dope will work but Teflon tape is a lot cleaner.
No, fittings go over the pipe
No you cannot mix the fittings, the properties from the galvanised pipe will weaken the properties in the brass. It is always best to either change or repair a pipe or fitting with the same material.
That would depend on the temperature "Temperature pressure relationship" and the fittings used
Certainly look up Flagg Flow T.P Fittings T.P thread-less copper fittings will FIT on schedule 40/80 /120 Brass pipe Ideally it should be brazed but soldering will hold domestic water pressures Also many solder fittings ARE CAST BRASS rather then wrought copper
You cannot do that effectively without taking the fittings apart,
Not sure what you mean by pressure fittings. Threaded fittings for steel pipe are pressure fittings. Steel pipe with a 3/4" O.D. is known as 1/2" I.P.S. (Iron Pipe Size) pipe.
If Teflon tape isn’t effectively sealing brass fittings, you can try using pipe joint compound or thread sealant specifically designed for metal fittings. Another option is to disassemble the joint and inspect the threads for damage or debris, which may prevent a proper seal. Additionally, ensuring that the fittings are tightened adequately and considering the use of new fittings may also help resolve the leak.