A slipping belt should not cause a sticky clutch pedal on a Greenfield: sticky clutch as far as I know is caused by worn friction surface on the clutch, which increases the required clutch pedal movement beyond what the linkage can achieve.
The remainder of what you said had happened is consistent with a belt being worn out, probably mostly due to that seized idler, though belts do not last forever in any case. So, if you have both a worn/shredded belt and a sticky clutch, replacing the belt is the first step but very likely not the last.
For future reference, it is very desirable to find the belt size, rather than the Greenfield part number, so you can buy one over the counter from your local industrial supplies store next time. It should be cheaper that way. As you may know, installing the new belt involves pulling the tensioner (the horizontal pulley adjacent to the two clutches) all the way back toward the clutch shaft.
Removing oily subdstances from the center square part of the clutch shaft is desirable, but after doing that you do have to keep the shaft from corroding and jamming the clutch. I suggest you find an industrial product that uses a highly volatile hydrocarbon as the delivery vehicle to coat the shaft with graphite or molybdenum disulphide powder, then dries up to leave just the powder on the shaft. Long ago I used to use a product called Drislide for that purpose, but there are probably others (and Drislide may no longer be made).
