I'm in the same case than you, with the same questions, exept that I have a mutewatch instead of iwatch.
One night I did 2 lucid dreams, because my watch was on the bedside table, and I've probably heard it while REM (I didn't notticed the sound) but my unconscious brain made the association, the link between the single short vide of the watch, and mind awareness.
I use one short vibe every 20 minutes during the day.
At night, I turn off the automatic mode and program the watch to vibe on 6h, 6h20, 6h40, 7h... to hit the REM spot.
I'm still beginning in LD, so I can't tell more about my experience.