John,
That is seemingly a body of evidence against a cooling system problem. But...

 FWIW here is my observation based theory on WBX water pump failure. It
helps if you are looking at the insides of one to fully grasp this. The
efficiency (for lack of a better word) of the pump is dependent on the
clearance between the edge of the impeller and the body of the pump.
Compare a new and bad pump and you will realize what I mean. That gap
widens and you are going to lose head pressure, which I think is
especially important in this setup. It's conceivable that it wears
rapidly when new and slows down as the gap widens.

I am also working on an oil cooler setup.  There are some pictures at the
bottom here: http://neksiwel.20m.com/photo_1.html

Click to enlarge. Have yet to plumb it in to the engine. Got to get a
round tuit.
--
Ken Lewis
http://neksiwel.20m.com/