One of a few things possible and you've mentioned two of them (stretched cable, but wouldn't just keep stretching that quickly to cause more issues that quickly) and rust/ flushed-in crap causing the plates to bind together.

Something you may need to look at is the spiggot bush sticking to the input shaft? You may have flushed some rubbish in there and it is now trying to all spin at the flywheel speed.

I'd still have money on it being what you're suggesting it already is. Clutch kits for them aren't expensive and they only seriously take around 2-2.5 hours to do. The other option might be to temporarily rig a clamp into the cable to give you more clamp/release (depending on which way you look at it, bearing or clutch face), put the gearbox in 4th and start the car with the clutch in.

Regards,

Dave