I am having similar issues with summer rear tires on my P85+. Worn: set #1 - 7,500 miles (inner only, plenty of thread left other wise), set #2 - 1,700 miles, set #3 - 2,400 miles. The issue is that original set-up did not allow rear camber adjustment, while some cars were getting to much of the negative camber from the factory. At some point later Tesla Motors re-designed bolts that allow for the adjustment of the rear camber. Unfortunately these re-designed bolts allowed rear camber to drift out of specs in a way original non-adjustable bolts did not. According to my conversation with regional service manager Tesla Motors are currently finalizing the design for the third generation bolts which will allow proper adjustment of rear camber, while able to maintain the set alignment without it drifting out of specifications over time. This problem was elevated to the very top and fix is coming soon.
In the meantime, I suggest contacting the regional service manager, he is real pro, and is totally aligned with the mission of Tesla Service, as articulated by the top management.
When I had a problem with service, I just send them summary e-mail and asked them to forward it to the regional service manager (His name is Lenny Peake). He called me the next day. PM me if this does not work - I have his cell number, but reluctant to post it.
I am confident they will make it right for you. TM paid for my two replacements and assured me that they will contact me as soon as the new adjustable bolts are ready, and replace the tires as necessary at that time as well.
Incidentally, I also swapped my winter 19" with Blizzacks by myself, and did not have any wear issues with them. The reason is that they have higher and softer sidewall than the summer tires, and, in spite of significant rear negative camber, conform to the road much better than summer tires. As a result winter tires distribute load evenly across the tire, while summer tires do not, riding on the edge which results in a well known wear pattern. The reasoning that was given to you by the NY SC is just wrong.
Good luck.