I have read again the entire topic. Judging by your description something wrong happened before or during the replacement of the previous clutch (the one that lasted only 5,000 miles). I'm thinking of driving style (clutch riding or tyres burning) and/or clutch alignment issues. Any or both might have caused severe flywheel wear or glazing. Any new clutch couldn't cure that.

So, gearbox replaced; flywheel skimmed; 4th clutch; new cable and (touch wood) it seems to be driving perfect!

What is your final opinion about the cause(s) of the slipping?

I believe it is as you suggested; glazed flywheel. When the clutch was first replaced at 55k miles, I think they didn't skim the glazed flywheel. I put a new clutch in and did the same. I believe it was the flywheel at fault although on visual inspection it looked acceptable.