Laurens, do you know if Alain replaced the pinion gear, crush sleeve and shims? If so did he manage to find a supplier for them and some part numbers? There appears to be a new pinion gear in the top photo. If the parts are available it shouldn't be too difficult the set the bevel box up correctly to hopefully run quiet. The bearings, particularly the forward one, don't look brilliant in the photos.
Reading back through the rebuild "Bwitched" did on his BB last year, his problem was caused by a lack of preload of pinion bearings, Alain seems to have had the opposite with them being too tight. Makes you wonder if Quaife are setting the gearboxes at all or just assembling them, the latter is much easier, quicker and cheaper. This would explain why some are quiet and others not.
I would expect that a pinion shaft fully setup would have a turning resistance of around 16 to 18 inch pounds to turn it. The size of the bearings makes a difference in how much torque is required to spin the shaft. This would be when the nose cone assembly is out, not in the case with the ring gear in tow.
On Ford 9" rears we used to call it the rule of thumb. If you could use your thumb and index finger to spin the shaft, it was probably quite close. Getting a torque wrench to measure less than 20"/# is not the easiest thing to find.
The front pinion bearing looks fried to me. I'm sure it would not have lasted much longer.
Speaking of BB's, did we ever find out what exactly seized up in our Texas friend's BB? I remember he said that it was the first time his wife had driven the M3W and at around 60 mph the rear wheel locked up. Obviously the box seized, but I don't remember what the exact cause was.
