RE: Fold down GoPro on bottom of fuselage
Cubs already have landing gear down there. It already creates a bunch of turbulence and drag. If you've ever messed with a Cub with scale shock absorbers down there, you probably didn't notice a major difference from a Cub without. There is a difference, but nothing to worry about.
Put the units CG straight under the a/c CG and you won't have that to worry about.
I've done some filming from above and never had much use for straight down. Sideways is way more useful.