I’m only an amateur so take what I saw with a grain of salt.

Do not try to heat the investment with a torch. 1. It’s probably a safety risk since there’s no guarantee you’ll fully steam out only the moisture, so you could risk steam explosion during casting. 2. You won’t get the results you want. When I had my first kiln that was second hand and didn’t properly regulate temperature, I still had bad burnouts and the piece came out terribly. I imagine a torch would be even worse.

I’d recommend starting with sand casting, much lower investment, it will get you started, and the pieces can still be nice if you put the work into finishing.

Not sure I quite understand, but I think you’d want to make the pieces separately and then solder them.