Alternate micros() extends millis() using ARM_DWT_CYCCNT
> Also makes unused_interrupt_vector() weak to allow user override like Teensy3

When you do a timer.begin(650) it actually triggered 652 ms from your current time.  Did minimal update to see if delay time < 2 set to zero else decrement by 2...

Two issues:

if you called begin or begingRepeating while still in waiting queue, it would create circular state (code did not set state to waiting)

If you called begin on one in active list, it would first remove it from list, but did not update the timings for items left as the timing all rely on items before it

When the run time issue adds an item back in to the list, it adds the item at the appropriate time, however it does not decrease the time of the item after where it was to take in account how much time the new item will take before we that next item starts to process it's _ms time