I respect that you are working through the problem in a logical manner and believe you are on track for finding a solution.
That said, at least in my opinion we are looking at the problem from a different perspective. What we know is that your engines overheat in certain conditions and you see air or foam in the strainer.
While it certainly could be a design problem it could also be an
equipment problem.
Rubber impeller vane pumps are a pretty good choice for the
raw water cooling application. They are simple, robust, and if they are in good condition they will draw an amazing amount of air, foam, or emulsion through the system. Simply, they are very good at self priming.
As the pump wears it becomes less efficient reducing its ability to draw these burps of air or foam. This could witnessed as air or emulsion in the sea strainer.
Pump wear is normally visually apparent during
inspection, but not always.
Most engine manufactures have a recommended guideline for rebuilding raw water pumps (bearings,
seals, impeller, inspect shaft, faceplate, not just impeller). In most cases it’s around 1000 hours. If your pumps have 1000 hours on them I’d start with an
overhaul. If it fixes the problem you’re golden. If it doesn’t, it needed to be done anyway.
Good luck finding a solution.