How do I Dunham a mainsail that is REALLY JAMMED into the mast?
What's a Dunham?
I don't claim to be an expert on furling mains, having only 2 seasons with them, but on the rare occasion I've had an issue unfurling due to a jammed sail I've re-furled carefully and slowly to ensure what's in the mast is wrapped as tightly as prudent without putting undue stress on anything (e.g. using an electric winch handle). That usually helps give me enough room to gently tease out what's wrinkled.
Alternately, get a rigger to fix it to ensure minimal damage is done, and then to help you figure out what's causing the jam and fixing it so it doesn't happen again. Either way, I'd definitely focus intently on what's causing the jam after you get the sail down. Lots of info here on what to check.
Best of luck.