Solved!

Wanted to post an update in case anyone in the future has this issue and their Google search leads here.

Threw in the towel after testing continuity of the ECT sensor and the wiring looms. Both were replaced, and it still threw the P0115 code immediately after clearing. Dropped it off at Toy-Lex in Webster, TX, and got the sense JP, the owner, knew his stuff. He let me know they narrowed it down to the ECU. While uncommon, it's also not unheard of for these to fail at random. I gave him the green light to do what he needed to do and replace it. I could have done it myself. It's an easy enough job, but I wanted to help out a small shop. After the ECU was swapped, everything was perfect.

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