With regard to dolphin kick, whole body dolphin, etc.
If you can physically do it but you can't find the right feel, try to think of your shoulders line.
People say "generate pressure with your torso" and getting the feel of that ain't that easy whereas moving the shoulder line is easier.
If you move your shoulder toward you back it flattens you back, and forward (closer to nipples to make it clear) it flattens you belly (torso).
There is clearly something about profiling but moving the shoulders also drive the subtle back movement.
Other than that I read the same thing as you did about butterfrog it is legal.