You mentioned just about all parts of the system except for the radiator. How old is the radiator? When was the last time it was cleaned/rodded or replaced?

BTW, as your t'stat experiment showed -- lowering t'stat temp NEVER solves an overheating problem. If the system won't reject enough heat to keep it below 200F-210F, how would it reject the ADDITIONAL heat needed to get the operating temp down to 160F or 180F. And the post about coolant moving too fast without a t'stat to allow cooling to occur in the rad is incorrect. The more coolant you move through the radiator, the more heat rejection occurs, even on a radiator clogged with deposits. You may not get enough heat rejection to keep the engine cool - the more flow, the more cooling.

ASSUMING t'stat is working correctly (sounds like yours are) and you don't have an internal engine problem (head gasket, timing, etc.) -- if you're overheating you're not rejecting enough heat. It's either because you're not moving enough air across the radiator or the radiator isn't capable of rejecting enough heat.

What happens to your temps when you're just cruising down the road at 45-55 mph?
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82 242 6.2L coming...; '15 Honda Fit