Problem Solved (fingers crossed for a while)
The only thing that made real sense to me was a temperature difference. When the soap hits the mold it is instantly cooled in the corners and when the sap process starts the corners never get as hot as the bulk of the center.
Unfortunately I'm not sure exactly what fixed it. I have a bad habit of changing several variables at a time instead of one at a time.
What I did:
Used a heating pad the size of my 3 molds, 14x20", set on top of a moving blanket.
I preheated the mold to about 95 degrees inside the cavity. The blanket was set on medium which is supposed to be 126 degrees.
I reduced my water by 10% which theoretically would make gel phase more difficult, but it was taking forever to bring to trace no matter how much stick blending I did.
I dropped my mixing temp to an exact 120 degrees from around 124-135 for both lye and oil.
I usually mix to light trace. This time it got to light trace reasonably well, but continued on to medium trace, like thin pudding, still easy to pour. It took a surprisingly long time to get from light trace to medium trace.
I covered all three molds with 1/4" particle board cut the size of the 3 molds and taped it up. My mold is made to use sliding plastic lids and they kinda suck as I fill to the top and tend to get the lids stuck when the soap expands. If I don't tape it up heat will escape through the holes that would normally be covered by the plastic.
I put the heating blanket to 136 degrees, set time to 1 hour and covered the entire contraption with another moving blanket.
Interestingly, after 1 hour I checked it and it looked to be gelled completely, at least on top.
I was almost ready to settle for the imperfection and was extremely frustrated. All things considered, about 15 hours and 140 lbs of tallow is not bad for perfection.
