I'm not going to pretend that I've digested every word of this thread - actually - I've read little except for a couple of posts you've made on page 4.
You mentioned some spots on the radiator being cooler than others. You mentioned temps were ok B4 changing out the radiator - you changed because of a broken neck.
Seems to me the radiator might be the problem. Is it a Behr? I personally wouldn't use any other brand. If the neck is just cracked in a spot, but will still hold a hose, try reinstalling the old radiator and see what happens. I'm not suggesting this as a permanent solution, but if the old unit is usable for a "test", why not give it a try?
I too had an overflow out the expansion tank problem. Thought it was the t-stat. Changed it. Still overflowing. Turned out to be a weak seam in the tank - wasn't holding pressure. I realize that you may have addressed this already - again - haven't read every word in this 4 page thread.
My 2 cents.