While I've never encountered that problem of stuck cylinder head gaskets to the
extent described here, I'm wondering if one of the cylinders is being brought
up to top dead center with the valves closed, and then pressurized to say 200
pounds per square inch with air pressure applied through a spark plug screw-in
pneumatic connector, just as a method of adding a little "convincing" helpful
parting pressure.   I assure you'd do something as obvious as that, but maybe
my thinking is showing itself to be "different" again.

Just a thought ~
John