I agree with everything in the OP actually, minus one thing.

The auto attack interrupts bit.

I think it would be better if instead of interrupting someone's cast from auto attacks, they should not interrupt but instead have a chance to slightly set he cast back.

For example an auto attack procs this on a black mage who is 1.4 seconds into a cast, and it sets it back to 1.1, or 1.2 seconds.