first of all, I don't know all technical word for all these parts...
Mine fails (2 years ago) because the inner part of the bearing on the of the pinion had slide a little bit and had started to grind the diff case (I was told if happens frequently)
Resulting in an oil pollution and the bearings "beginning of the end"
However I was able to ride several hundred KM without being worried
Leak appears right after a highly muddy weekend (GS Challenge)...
Maybe the leak had nothing related to the diff "on his way" failure, but at least it helps me see it before it turn into drama.
I replaced the part myself, but I bring the diff to be adjusted by a BMW mecanic master.
IMHO, the preload adjustment is the key to have the diff lasts long.
I even think the preload should be re-adjusted during the bearing life
Sans un sou mais plus riche qu'avant