Are the SqD breakers actually "classified" for a Murray panel? It might be worth checking that out. I know SqD had a real dust up with Cuttler Hammer over the Challenger breakers in SqD panels and it might seem a little disingenuous for them to go back to U/L and look for the OK to put their breakers in someone else's panel.
Just because they "work" and they probably do, doesn't mean they are legal.
As for your other question about the GFCI in a 240 only panel (no neutral) you would have to look at the manufacturer's instructions. That set up was pretty normal with computer room panels. They did not bring the neutral to the panel. The insulated, grounded wire was reidentified green and it was the isolated ground. I suppose you could hook the white wire from the breaker to the ground bus and the GFCI would "work" as intended but the instructions should be your guide.
You could go the other way and use the 30a in the main panel and exploit the tap rule, using the 20a as down stream overload protection. That would also provide GFCI protection to the feeder.
