We ended up at cross-purposes, aldot. Because there was a thread with no nut in your picture, I thought the inner part of the clutch plate was a retaining nut (there had to be something on that thread on the shaft, and you didn't mention having removed the nut) so I advised you not to hammer the clutch off the shaft while there was a retainer holding it on. The way you did it in the end (remove the retaining nut then support the clutch plate on both sides, close in to the shaft, and give the end of the shaft a light knock with a mallet) is the correct way to do it.
The sticking of the shaft in the clutch plate stems from the same problem mechanics traditionally have with the clutch plate driving spline in car gearboxes: it needs to be lubricated because the clutch plate has to move along the spline each time you declutch, or the clutch will bind and grate the gears when you shift them. However on the mower, the whole thing gets coated with dirt, and wet lubricant acts like a dirt magnet. The combination of dirt and oil makes a cutting compound quite similar to the stuff you use to lap valve seats. That is why I think I'd prefer dry graphite on your dirty clutch shaft, though I tolerate a smear of engine oil on car clutch shafts because they are enclosed and stay somewhat cleaner (they only get coated with dust from the clutch plate).
