First thing I'd do is a clutch calibration if you haven't already. Instructions for doing so are in the owner's manual. Make sure the oil is hot.
I don't know how you're determining that one clutch is not releasing soon enough, but I suspect you're overthinking that particular angle. If you're looking at both F1 and F2 current ramps with a scope, you likely will see both engaged at the same time for a brief moment. Since it takes time for a clutch to actually "bite" once it's engaged by the modulator valve, you don't want the off-going clutch dropping too soon or else you end up with a huge hesitation, which is not good under a heavy load.
Understand also that the 9-10 upshift and the 8-7 downshifts will always be the roughest on that particular machine. Having said that, it shouldn't stall the engine while doing it.
If it was in my shop, I'd be comparing actual clutch pressures with the current signal while doing a calibration, for both F1 and F2 clutches. I'm guessing one or both are sticking and not modulating like they're supposed to.