1: if the coolant is relatively new and no more than 50% coolant, it shouldn't be a factor. I stated that water has better heat capacity and lower viscosity than glycol, but it's not as great a difference as I had thought, and the higher boiling point of the glycol could easily offset the lower specific heat capacity, especially in high ambient temps.
2: Yes, Nige has mentioned on other threads that bigger radiators like this can have enough film to slow heat transfer but not enough build up to block the flow, so that the deposits will reduce the temperature drop just like a dirty outside or low air flow will. I've never seen that, mostly because blocked radiators I've seen are mostly smaller than this, are blocked with lime scale that blocks the top of the tubes, and almost never do anything in 40C.
3: I'd guess running it with rainwater once or twice to confirm there's no sediment, then a flush chemical would be plenty. The cores are all new and it had new coolant at that time, but previous rust. The most important thing to learn from draining the coolant and flushing is how much sediment comes out, back flush the radiator if you can't see the top of the tubes, check the oil cooler per TCT.
JAJ, I agree with your thinking about the temp drop through the radiator. If the exhaust is leaking at all into the pusher fan, and the fan shroud is leaking, the high ambient temp is already your limiting factor, so that would be enough to make your cooling marginal. I think your operating temperatures are marginal, but as long as you're not losing coolant, and the cap holds enough pressure to keep the system above the boiling point, then it's safe. But you'd be more comfortable if the temp stuck exactly at the full open temp of the regulator, and if it climbed above that you knew you had to blow the dirt out of the radiator.
If the coolant flow was reduced because of a worn impeller or a stuck closed regulator, then the temperature drop across the radiator would go way up. Just like if it was operating at -40, the regulator would barely crack open before the incoming -30 coolant mixed and reached the regulator to shut it closed again.
If the coolant flow could be doubled from your existing operating conditions, then the temp drop would be roughly halved, even as the radiator would expel slightly more heat because the bottom of the radiator would be hotter therefore the air going through would be heated more. If there's only 4 degrees drop to start with, then doubling the coolant flow will raise the average radiator temp 2 degrees, so it won't dramatically increase the heat rejection quantity. 75C air to 100 vs 102 average coolant temp. so 27 instead of 25C temperature difference from coolant to air.
If we guess the average air temp going into the radiator is 60C from heat off the engine, and the average air temp coming out of the radiator is 90C, then doubling the airflow would drop that 75 degree average air temp to 67.5, so a 32.5C difference between the coolant and air, vs 25 for original airflow, or 27 for original airflow and double coolant flow. You can see the airflow is critical the higher the incoming air temp. If you have a dial probe meat thermometer that reaches through the cores, that could measure the air temp before and after the radiator in a few places and figure out what we're working with. Obviously you won't double the airflow, but you might improve it a enough to make a difference.
"I would have though the slower the fluid moves from top to bottom the closer the temperatures would be between the top hose and the bottom one." No, assuming the airflow is the same, the longer it takes the coolant to pass, the more it will cool off, approaching the air temp. That assumes the flow is even also. One side of the radiator blocked and one side open will dramatically reduce the temp drop and heat rejected. Blocked tubes spread evenly over the radiator won't make as much difference because the fins will transfer heat better to the blocked tubes and continue to reject heat from all the radiator surface.