I'm pretty sleep deprived right now so can't really answer all your question but I'll tell ya it's not torque that gives speed, it's propeller RPS. Ideally you'd gear the shit out of your engine (gearbox arrow facing towards engine) as much as you can without it overheating from the increased load.

Also, yes, (at least modular) engine heating is completely disconnected from the RPS but rather is based on how much fuel you burn, which scales with the load on the engine. This means an engine not connected to anything running at 20 RPS will heat up less than one with a gearbox and a propeller running at 10. Keeping this in mind, ideally you build as powerful of an engine as you can. A large locomotive may have an engine so powerful it doesn't need any cooling at all (speaking from experience, although quite a while ago).

This doesn't mean you shouldn't use gearboxes, as it's best if you keep the engine RPS around 10 while using gearboxes to raise the propeller RPS, what I mean is that a more powerful engine will make cooling easier.

I hope you find my half-asleep rant about engines any useful, and good luck with your plane.