Good summary.
I'd simplify it thusly:
Engines run on expanded air. Burning fuel expands the air. Not all the heat from the burnt fuel turns to power that's why exhausts are hot.
Torque is based on how much expansion you can get and is a function of rod throw and piston area (diameter) and total expansion (roughly related to compression and time).
Horsepower is how fast you can get through the power cycles.
If the head flows ****, you'll still make torque (maybe more because the air expands for longer since it can't get out) but you can't exchange the air and so can't rpm and thus **** hp.
Flow is dictated by engine architecture as RB says. Some architectures can't support enough valve, or port, or compression, etc due various constraints. Take two engines of the same displacement, and both will make similar PEAK torque numbers, but the one which flows more at the RIGHT time will keep making torque as the revs increase and thus make POWER.
When it comes to flow, there's a practical limit to what can be done with poppet valves. Airplane engine mfgs knew this decades ago. Go look up sleeve valve engines from the 40s and prepare to have your minds blown.
