Just to wrap this story up, Amperetime didn't give me a hard time at all about returning the batteries. After I shared the details they sent me UPS labels and issued a full refund after they received them. I didn't mention this thread, but I did share links to the videos. Kudos for Amperetime for owning it and stepping up.
So in the end I'm out nothing, save for the hassle and hauling them down to UPS. Amperetime however is into it for shipping 50# x 4 x both ways which according to the UPS tables is about $1000. The total original sale was $3249 which works out to $317/KWh, shipping included. Ouch.
fwiw, I did some more testing before sending them back, varying SOC and load vs. charge. Basically any time the current reversed flow from charge to discharge the BMS would cut out, sometimes with strange effects. When wired up in parallel sometimes I'd end up with one battery shut down completely, and occasionally a BMS would just go nuts and start oscillating the output. Bottom line, this product isn't ready and should be avoided until they figure out what's going on.
On the bright side, I also ran a quick load test and with 4P x 200Ah I got 830Ah. Mostly that was at 100A but at the tail I ran a light resistive load since the LV fault on my inverter tripped before the BMS shut off.
Meanwhile, I bought a 304Ah Trophy as a replacement and have been super happy with so far. $/kWh was about the same in the end, but much better product and obviously preferable to managing a string. Trophy has built-in heaters too, which I will need occasionally in my application. I will insulate but hold off on adding additional heaters until the data says that makes sense. Eventually I'll add more units as well, probably 60kWh total.
