I think he may have called me and left me a message but I will answer here as well and will call him back when I get off work.
First thing I would do is drop the cross to 62-64% and change the Left Rear pills to OUT. Get on a tick higher left to 58%. Nose seems to be good with that type of tire. Front stagger needs to be 1 1/2-5/8" and the rear 1 1/4-3/16". RF toe 0 and LF toe 1/16" out. Seat needs to be 9".
The issue you are having is running so much cross on a tire that is so forgiving. I am guessing he is pushing apex off. With his seat height a little higher then normal he is transferring weight to the rf way to quick as he puts any type of wheel input into the kart. Just a little thing to add to the notes....... If youre on a tire punching that soft with a soft side wall, no way you should be that high on cross plus being the pills are IN. You load the rf so quick it has nothing left from the apex off as I like to use the term (Used up the right front). Make sure the rf is setting stock as from factor. I believe it is just the one bigger wheel spacer on the inside, 1/4" on the inside of lf, 3/16" off frame on rr and 1" off frame on lr.
How far is the rs seat strut off the inner motor rail? It should be around 4 3/4"
I did not call and leave a message I plan to call as I think it will be easier, push started on entry would not turn at all, ONLY ran that much cross BECAUSE Factory told him 70% regardless of tire we figured it would not work.
That info 1st run was a 1/5th mile Tri oval tight turns, Increased rear stagger between runs, decrease RF camber 2 flats NO Difference.
2nd run 1/8th mile Momentum sweeping turns, showed up with less cross 60, left RF camber at 2.5, rest same still 11's this track requires same tires soft 11's with lots of bite, SAME results would not turn, Increased LF camber 2 flats, put tires on that just finished 2nd in clone 340lb with the Triton which helped slightly for the feature still pushing 3th off the leader.
3rd run back to the tri-oval with a different driver racing it who weighs much less lite 65 lbs shorter small upper body, re-scaled moved castor to 10 & 13, 2.5 RF camber, .50 LF camber, 57% left, 47% nose, 60,4% cross, pills LR out, RR in, hot laps it still pushed, increased rear stagger, Increased LF camber 3 flats got ride of push actually little free, No speed No forward drive, for 1st feature took some of the LF camber I added I took back out and right back to pushing and still no forward drive on straights. for 2nd feature added RF camber & LF camber 2 flats, increased rear stagger a little, started out half decent would turn better only lasted a few laps back to pushing and still no forward drive.
So now after know the rest of the story and were already close to your suggestions above, What do you suggest ?