Yes, all those things are factors.
We're just dissecting the red vs yellow brass side of things and figuring out how much impact that would have.
Optimal fins per inch really is a bit open as well, as like pumps, different fans respond differently to different levels of air-pressure resistance.
Axial fans would indeed prefer a more open fin spacing.
Blower type fans (
like this one that I picked up) can provide greater pressure and will work better with higher FPI radiators. The radiators I use are 24FPI and the blower I use is barely affected.
Bill (unregistered) is more familiar with air pressure-drop relationship with radiators than I am though.
The heater-cores I use are constructed by Nippon Denso (Toyota subsiduary) and most (all?) of the qualities you describe are present in their design, including homogenous flow entry, low flow restriction, and concertinaed slits on the fins themselves disrupting laminar flow, and despite using 24FPI, still have a fairly low air-flow restriction because they are thinner.
I like the radiator's you are making, but it is also wrong to assume that all heater-cores are not made to the same level of optimisation that you are talking about. The Nippon Denso cores certainly are.