OK, I'll address the other half of your statements.
Isn't that a bit of a stretch?
You assume that if X is true, then Y is true, then you assume that if X is false then Y must be false, therefore saying X=Y. It's an easy assumption to make, but that doesn't make it true, especially if we are to believe that the conditions at GZ had never happened before. I call faulty logic.
Going back to the OPs questions.
Does molten steel prove or disprove thermite? No. It only proves molten steel.
If so how much thermite (a self oxidizing agent) is needed to still be reacting six weeks after initiation? No idea, I don't know anything about thermite, despite beachnuts assertions.
Why are other avenues of steel melting (such as in a rare natural furnace effect) dismissed? Not dismissed by me, happy to hear about them.
Since liquids follow the path of least resistance they will pool at the bottom of the bathtub. How was the steel that solidified there removed and where did it go? Faulty Logic, they will pool at the lowest point they can flow to before cooling, this does not always mean the bottom. Where did it go? China most likely. How was it removed? Dunno, the area was restricted, ask the ironworkers or the guys with the big machines.