No, only the STOP footswitch can stop and clear a loop.
The 720 Looper is not compatible with the 45000 foot controller. The 720 can be controlled by the FS3X (or equivalent) to control bank up, bank down, or undo-redo functions.
500ms
No, 500ms is not short enough. However, its possible to record a very short silent loop initially, followed by overdubbing constant tone(s) to get a similar pad going.