Pusher Fan Advice?

The speed of the circulating coolant is not a factor unless it's not moving (thermostat closed or restriction somewhere). Since it's a closed loop system with one pump, the velocity of the coolant is the same in the engine as it is in the radiator. It's on both sides of the equation and therefore cancels out, leaving surface area vs. surface area. The surface area in the engine is basically fixed, as is the radiator. So the only two variables you can really change is the amount of heat going into the system or the amount of airflow through the radiator. But even then airflow is only a factor if it's way too slow.

Since your temperature begins to creep up in low RPM/low airflow situations, it seems logical to increase airflow a bit. As you indicate, getting the car moving a little faster brings the temperature in line. If you put it in neutral and idle up to 1500-2000 for a minute or so, does the temp come down? If so, then a booster fan might help. If not, then it may be a matter of getting the heat out of the engine compartment, not just the radiator. Ideally you would monitor the temp in the upper and lower radiator hoses (and underhood temp) and see what's really happening. If the underhood temp climbs in parallel with or faster than coolant temp, a bigger fan might not be much help

The drawback to pusher fans is they pose an airflow restriction once you are moving which gets worse with vehicle speed. No fan manufacturer is going to recommend dual (push and pull) fans. The best solution would be to remove the 7-blade and install an electric on the engine side with a manual override switch. Or better yet, avoid parade speed traffic.