Current thoughts are differential failure or internal gearbox failure.
I don't want to worry you but a mates car had the friction plate fitted the wrong way round and ended up needing a new gearbox. Fortunately as it was fitted by a garage they were liable and had to pay for the new box.
Get the box out and test it on the bench. remove the diff completely so that you can see the output gear and turn the input shaft by hand, change into all gears and observe that the output gear is moving. then check to ensure the diff is still complete, no broken planet gears etc.
I am assuming that when you removed the clutch the 2nd time you unclipped the release bearing and re seated it on the fork?
Alfa GT 3.2 V6 - Daily Driver.
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