I may be continuing to miss the point, but ...

I still think TMR2 should be a high priority interrupt.
Except, it should be ASM, and ONLY collect the data required to calculate the error.

The PID and motor control should be running in the main loop, using the error info from the interrupt.

If the PID happens to miss an update, it won't matter that much because the error will still be updated the next time. And it's not like the PID loop can cancel the error on each update. It takes time. But at least the actual error measurements will be consistent at much higher frequencies.

Obviously, the "I" time period won't be constant, but I think they'll be close enough that you won't notice a difference. IMHO
Then the PID loop can run at it's maximum rate, while the interrupts run at their maximum rate, without interfering with each other.
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