Nope

Flying straight and level has nothing much to do with the airfoil, flatbottom or not. And semisymmetricals don't demand any kind of trim from the elevator if the airplane was designed correctly. The size of the horizontal tail, how far back it is, and it's angle of attachment to the fuselage (the angle of incidence) are all designed to adequately control the pitch of whatever airfoil the designer chose for the airplane and to provide adequate pitch stability. The designer designs that horizontal tail to work adequately and efficiently, and having to trim the elevator suggests he didn't do the design correctly. Yeah, you might have to trim for a speed other than normal cruise, but thats an exception. Models and fullscale planes have a design speed where trim isn't needed.

There isn't a best airfoil for flying straight and level. The major difference in your three types of airfoils is how much weight they can carry and how efficiently they do it.

Wing loading is a major factor in the flight characteristics you're attaching to profile shape and dihedral is another.

You really can't attach such general flight characteristics to profile shape. If you want a good understanding of how models and full scale airplanes fly, "BASICS OF R/C MODEL AIRCRAFT DESIGN" by Andy Lennon is an excellent book. It's cheap, covers EVERYTHING and does it very, very well for guys who don't want a degree in aeronautics but who do want to understand it all. You'll find out one thing for sure. That there are a number of important things and not a single one of them "does it all" on how the airplane flies. The book might also catch your boys attention when he starts reading. And until then, it'll give you answers to some of the questions he's sure to ask about airplanes. He hasn't started asking about everything yet has he. Wait until he learns the words "why" and "what".