Hi Mark,
You use Master & Linked Video when you want a single video to run over multiple slides.
What I think you are trying to do is run a short video over a single slide which has a duration that is longer than the video.
By slowing down the video - I set the Speed to 50% - it gives you a duration of just over 12 secs which is long enough to cover the side & the Transition.
I also put a very soft blur on the video.
Depends how visible you want it to be, but on my laptop anything less that 30% opacity is not really visible.
May be different on your Mac.
If you want to keep the rain at 100% speed then you need to add a second copy with an Offset of 6 secs (the duration of the video)
This 2nd copy will then start to play as the first copy runs out.
Both are single video clips.
If in your final project the slide duration is longer than 12secs then simply add more copies with the appropriate offset.
Hope this helps - but email me if you need more.
Jill