You're posting this in the Model 3 forum. If you want to get responses from people who are more likely to know something about your problem, you should post in the Model S forum. I haven't seen "several owners reporting exact alerts/warnings showing up on the screen since April this year" in the Model 3 forum, nor have I seen where "Lots of Teslas seem to have exact failures in the last 6 months and it can't be a coincidence." I don't know where you're getting your information from, but it doesn't seem to be an issue with Model 3s.
Also, you state that you have a 2012 Model S P85. I don't know how many miles you have on it, but your car is nearly 10 years old. It's certainly not unusual for a 10 year old car to require some moderately expensive repairs or maintenance. Judging by the parts cost of $300 for a "rear drive unit and 12v battery", it would seem that you're getting the rear drive unit for free, since I'm sure that a new drive unit costs thousands of dollars. This strikes me as exceedingly generous on Tesla's part since you are 2 years out of warranty and it is anything but a ripoff.