What seems odd is that your temps don't go down during highway driving -- which in turn tells me that the cool harness may be of little benefit if any at all.
How does your coolant look? Do you see any signs of carbon inside the expansion tank? The separation of the overflow hose is not an issue ... mine comes apart all the time and I just stick the inner tubing back into the other hose.
Coolant/water ratio could be the problem, so definitely check that, and throw in a bottle of water wetter. My mech told me that with a bottle of WW, it's safe to go down to a 70/30 water to coolant ratio with no corrosion problems down here in scorching hot and humid Houston.
Good luck and let us know what you find