On the pressure relief valve at the water heater?
Like Pex? I recommend strapping it to the water heater. I remember being ridiculed here for making that recommendation though.
Pex is approved in my area but your area may differ.
No sir flexible copper instead of regular tubing…
The soft kind or corrugated?
Not in this area ,recommend checking local codes.
My copy of the 2000 International Plumbing Code Commentary Section 504.6.2 uses the words “rigid piping materials” . I assume the theory behind rigid is that they don’t want someone turning it into a pretzel.
in the city it is not allowed, in county it is
just a local central Indiana jurisdictional tidbit for you. IOW, check with the gas company or local building authority