So, I spliced a flow meter into the return line of the battery loop, between the temp sensor and the degas line. It's very easy to get to with plenty of space around it. I also spliced in NTC temp senders on the inlet and outlet of the chiller block.
While the battery was over the cooling set point 97F+, I was seeing something like 6-7L/min of coolant flow and typically seeing a 10F delta across the chiller - but no useful battery cooling.
Setting degas mode (diverters centered, pumps at full) I would see ~14L/min through the battery loop. I tried pinching off the upper and lower packs to see if there was a notable change in flow, but didn't see anything to suggest either was blocked. Temp values from Forscan didn't indicate a large temp delta in the battery either.
Leaving my place last night, I had the idea to pinch off the hose that joins the battery cooling loop to the rest of the system. There's one near the chiller block where coolant can flow into the loop and one at the diverter where it can be directed out. When I pinched either, my flow meter dropped to 0.
The diverter shouldn't be sending anything out and nothing should need to flow back in, so I checked Forscan - shows the diverter is set to (and reading near) +55deg which is the orientation to loop battery coolant back through the chiller. I use Forscan to move the diverter and see no difference in flow.
The measured value is following the setting, but it seems that the actual diverter is stuck in the open loop position. Bought some caps, couplers and clamps, and physically connected the return to the chiller inlet. Start it up and can watch the temp fall on my ScanGauge.
I was able to just plug it in and let it charge on L2 last night without worrying it was going to spend the night cooking itself to death.
So there you have it, more than a year of cooling problems that the dealer and Ford couldn't troubleshoot, isolated to a bad diverter.
If you ever suspect you're in the same situation, turn your battery pump on to a low-is setting (25-30%) and set your battery loop diverter to +55deg, then clamp the line that goes between the battery loop and the rest of the system and watch your degas bottle. If the diverter isn't returning coolant to the battery loop as expected, clamping that section will cause it to start spraying out of the degas line into the reservoir.