NOTE:
I discuss this based on a reasonable compact URTOS that will fit a '51 NOT a full fledget large OD for a powerful processor.

Interrupt dis/enable controls if you respond to an event, not when.
If you disable an interrupt it will happen when you re-enable it (except level ints)

Interrupt priority selects which interrupt (event) is responded to, not when.
Indeed, it select that you want to handle it when the external event happens.

With a time triggered RTOS you choose when external events are are handled when you define when the task that will handle them runs.<>/i>
Nope It happens when the scheduler get around to letting you do it.

Erik