But I'm certainly not buying another Subaru after this ordeal
How is this Subaru's fault? They didn't service, or probably more correctly failed to service the differential. Clearly the cause of it failing was a lack of lubricant. Given there is no evidence of leaks the most likely explanation is that someone forgot to refill the diff.
But there's known problems with Niss-err-Subaru's CVT
The Jatco CVT used in Nissans is a completely different transmission to Subaru's. There is little evidence of systemic problems with Subaru differentials.
If Subaru gave a damn, they would cover their transmissions.
What was Subaru's response?