Sounds like a fun project.
On the rocker panels, just be careful when you pull them off - as I mentioned above. But I am very happy with the results it made on my car.
I think the reasoning behind not putting any sound insulation under the carpet is the fact that the battery - being so thick and directly under the floor - should act like a sound barrier, but the noise seems to just transmit through it, and Tesla is probably too cheap and too concerned with adding more weight to properly isolate it. There is no foam in between the battery pack and the floor, which would probably help (again another cost cutting move?).
In addition, on my car the rear floor section behind the front seats flexes and hits the battery pack underneath it, it literally sounds hollow and like a tin can when you press on it - Not sure if the new models have this known issue anymore, but I am sure it could have been solved with some sort of pad in between the floor and battery - just unacceptable in my opinion.
Interestingly, new Mercedes EQS sedan has an insulating pad attached to the battery pack on the outside facing the road, thus isolating the battery pack from the road. The car is super quiet.
I’m wondering if your technique would work on a Model X. When I get home I’m thinking about pulling off the rocker panels and copying you. But I’d better have some silicone glue on hand before I start. It’ll be my luck that I’ll destroy all those clips.