I do not think the analysis of this problem is quite right. I have a NSX water manifold here on my desk and the passages in it do not align with what you describe.
This is the flow through the manifold. Red is the hot water coming from the cylinder heads, from there it is either recirculated (yellow) or goes to the radiator (red). At first glance, it does seem like the thermostat sits in the flow of cold water.
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But if we look more closely, we can see that recirculated hot water is pushed directly over the thermostats wax element, to the point that very little cool water will ever come into contact with it. That hole there in the middle will always have hot water come through it in some quantity which is what is largely controlling the thermostat. The hotter the water coming out of the heads, the more the thermostat will open. (and the less hot water will be recirculated)
There is going to be some intentional mixing of hot/cold water here and that is necessary for the thermostat to properly regulate.
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Your solution also has a few issues. The first of which is that you are using the ECUs temp sensor (which monitors the hot coolant at the very end of the cooling loop before water leaves the head) to control the radiator fan. This makes no sense, because the actual operating temp for the C30 is about 85C which is 185F, which is where your fan comes on. The radiator fan needs to be triggered based on the radiator outlet temp, which is why the factory fan temp sensor was located on the thermostat housing directly in the return coolant path.
The 2nd issue is when you deleted your thermostat you also removed the metering device that controls how much coolant is recirculated. Coolant will take the past of least resistance, and without the thermostat there to control the amount of hot coolant that is immediately recirculated you just introduced a massive and potentially dangerous unknown to your cooling system as well as likely reduced its maximum cooling capacity.
My recommendation would be to put the thermostat back where you found it (well, a new one, since there's a chance the old one was bad and it's now destroyed), and fix your cooling fan activation issue, since that seems to be what started all of this. As far as the radiator goes, if you can get your hands on a MASIV setup that is the best option, otherwise double or triple pass converted Koyo radiators seem to be good.
The fluid flow path you mention is also backwardsish, the water pump pushes coolant into the block, where it then flows around the cylinders and up to the heads, and then out into the 2 legs of the water manifold. So the correct flow is:
Water pump -> engine -> thermostat/radiator -> water pump again
As far as blue vs green coolant go, both are fine as long as you change them at proper service intervals. IIRC the Honda blue stuff is good for 4 years while the parts store green stuff is 2 years.
