Thank you for the windows update. I hope you don't mind, but I immediately swiped a portion of your post as an addition to the Windows section of Black Arts. Prior to this, everyone has said that the frames do have to come out and that's the sort of additional complexity that makes people shy away from a very rewarding job ('hard work pays off tomorrow – procrastination pays off immediately').
As to replacing the gasket, note the recommendation in Tools and Spare Parts for 3M Spray Penetrant as a superior product for loosening stiff fasteners.
C&C 27 Mk IV