lets address some of these statements
wrong the additives in the gas attack the non gas rated oring breaking it down and in turn leaking / passing past the oring
if the gas line is above 130 dc temp wise i think you have bigger issues as the incoming service could be plastic, which as you know doesnt like heat/ hot environments and this will cause a problem before the solder melts
copper pressed can only be used on and upto 5 bar stainless can only be used upto 10 bar
as for soldered and your stated 15-100/50 bar tests, as you already know you cant soft solder past 75 mbar you need to braze in turn answers your second point braze will go well past press in regards to temp ratings and tbh you will prob find your copper tops out around 30 bar anyway