RE: Rocket assist airplane
There's been all sorts of stuff done, including a scale X1 dropped from a scale B-29. If the rocket motor stays with the airplane and isn't launched from it, and if the motor is not above G impulse, it will not violate the AMA safety code. If the motor is larger than a G, and if it's launched vertically, it is technically a High Power Rocket, and you can fly it at a HPR launch under their insurance.
There are lots of rocket motors available that are a lot bigger than Estes offerings. Not cheap, but with the advent of reloadables, not highly expensive, either.