Lots of misinformation out there.
Correct but ...
a. Pinion preload is not set by nut torque to an appreciable degree. It is set by the collar and shims on the pinion shaft behind the rear pinion bearing.
b. Pinion position is set by shims behind the bearing shells.
100% correct and the torque of the pinion nut will never affect pinion preload or position. A myth ...
c. Putting gaskets on the axle tubes has no effect on crown preload or pinion alignment at all. It only effects the depth of penetration of the axle splines into the spider
Sorry, incorrect, but there are factors involved. If you remove the axle tube that contains the diff side bearing and add a gasket then reinstall without adding shims behind the bearing cup (the same thickness as the gasket) you will have altered the diff side bearing preload. BUT if you rebuild the differential and set the preload based on the thickness of a gasket then yeah no affect as you say, but that is not how 99% of Alfa diffs are setup and in this case they were trying to remedy a leaky diff, not rebuild, so if there is now a gasket or thick gasket goo we have a problem that needs to be fixed immediately.
d. I agree 100% that if the seal is replaced, it is best to return the nut to the previous position.
Yes while contradictory to your point A, it is good practice to return anything removed back to exactly where it was.
e. You might try redline 90-140 NS gear oil to see if that helps. it might
f. Don't rebuild a diff unless you have the right tools, and a lot or patience.
g. Lash is a factor in diff noise as well as pinion position relative to crown gear.
Pinion position relative to crown gear was set by the factory and never needs to be altered. Just ensure the shims under the pinion to the first pinion bearing remain the same. New bearings are made more accurately made than we mechanics can measure, so just leave everything as Alfa set it and just set preload and backlash. This is where most screw up a diff rebuild and confuse themselves with pinion position relative to the crown wheel where you actually don't have to worry about it at all