Hello & welcome

I am sure that the fun driving will start soon after your initial set back.
As for "wise words" you went shopping at the "Harrods" of the Fiat car parts world. Nice to have new but you probably could have saved a lot if you had got on the forum first and requested a good used drive shaft and drive coupling flange. I assume that you are replacing both parts ?
If your mechanic is not so confident with the gearbox then the old aircooled Haynes manual can be got quite cheaply and covers a lot of the Bis mechanics. Or you could subscribe to the Forum and then get access to the full Bis workshop manual online.
I would suspect that the spline was running dry (not greased) , possibly damaged or missing flange to shaft seal and can also be agrivated by a dodgy wheel bearings or drive couplings.