I'll be succinct here - I think you're making this a "scarier" issue than it really is in reality. If you install aftermarket suspension on your tesla and it's proven that the aftermarket part caused the "issue" then you have no Magnussen Moss case. However, if Tesla cannot prove that the suspension caused your suspension related issue, then you have something to work with, in my opinion.
Did your M5 buddy say what was wrong with the car after he tuned it? Let me guess - he tuned the car, got over +100tq, +60hp whilst the transmission and everything else is stock...he ran it hard and something engine or drivetrain went wrong and tried to claim a warranty on his car. While i appreciate you giving us an example, this is not a good one. Coming from the tuning world with Audi's, Porsche's, Toyota's, everybody knows once you tune the car for more power under warranty, you kiss your warranty goodbye. In my opinion, your M5 buddy had a poor position and case for Magnuson Moss from the start.