All of these suggestions sound good and have worked for me other that trying a different seat. Still using the T11t myself. After watching the section of video where you start tipping up several times, I noticed your corner entry was quite early and making your apex a little early. This makes you have to hold the wheel longer into the apex where you are already back on the throttle. At this point the kart is gripped up and the extra throttle tips your mass over, taking the inside wheels off the ground. The driver behind you, who’s footage you were using, turns in a little later noted by how you cross the center line of his bumper. It appears he is already unwinding the steering where the kart is under peak load (on throttle, near apex). He caught up to you in the corner before with a similar line. He drove in a little deeper, got underneath you in the corner and drove out further as a result. By straightening the apex some, you can drive further out of the corner before reaching the outside of the track and carry more momentum as well.
I am struggling with T7 at NTK. Its a sharp hairpin left at the end of relatively fast straight. After the track was sealed, I found I was bicycling at the apex out. The track surface is very grippy at the apex. After making the chassis adjustments it got better, but was still lifting both inside wheels a little. I found when I turned in slightly later, got the kart rotated more before I was all the way back into the throttle, I was able to keep the kart more stable through the apex and the sticky part of the corner. When I tried a smoother arc toward the apex, I found I was too loaded up and lifting the inside wheels. This started happening as the tires came up to temp. I started playing with my line and the bicycling got better. Unfortunately I found myself doing it again when trying to defend the inside line later in the race. It ended up costing me the position when I tipped up trying In to save it, much like you did. In hindsight, I would have been better off sticking with my adjusted line and beating him to the inside of next slower corner.
Not sure this will help in your situation, but might be worth playing with it.