Greasing splines where clutch disc slides is controversial among mechanics.... some use graphite dust instead of grease thinking the dust wont sling off and slime the disc - others use NO grease at all - even worse they remove existing grease from the spline and assemble it 'dry'.

If rust has frozen the disc to the flywheel, you can bet the disc/spline is participating with the problem....... you might have to bite the bullet and drop the tranny to free it up.

Or if it was me, I'd first be crawling underneath with can of WD-40..... shooting penetrating oil through spicket attachent on aerosol can through bell-housing weep hole underneath - litterally soaking as much of the clutch as i can reach, knowing that penetrating oil will evaporate with little residue - so not to slime the disc pads.