Having rewatched that video - a previous poster was right - the person saying "I can't there are cameras" does appear to be off screen.
I doubt there's much to the garden party story. Harry has shown himself to be a terrible actor (see the Commonwealth service), and there's no hint of tension between H&M and C&C.
There's plenty of footage of M clearly not 100% of protocol over the years, as she hesitates, and looks to H for guidance.
Even she looks relaxed at what was one of her first public events post wedding.
I guess it's probably standard to acknowledge that some member of the RF draw additional attention at different times. Immediately post wedding H&M would be outshining all other royals.
So I think it's probably natural that things are arranged so others get their share of the spotlight.
As it's Charles's 70th, I can imagine it was engineered so that they showed their faces in a show of support, but then left so they weren't taking attention away from Charles.
The RF carefully time speeches/announcements etc so they don't over shadow each other, so this is only an extension of that.