OOOOOH, I didn't realize you were super dependent on this.
You can probably just kill the main dual breaker to do the no load test. (I'd start there for sure)
This PDF has some good info on firmware compatibility.
Interesting section:
Firmware compatibility in three- and split-phase systems Units with old and new microprocessors can be used together in parallel and three-phase installations without any problems. Combining virtual switch firmware (100 – 199) with assistant firmware (200 – 499) is not possible. It is important to always use the same, and the last, firmware in each unit.
The other thing I would do would be to stick a multimeter on each leg and monitor voltage during the testing. It could be helpful.
Any data point is useful.
I'm a network engineer, so here's my mantra to co-workers:
- Make a diagram
- Double check all your assumptions, check the basics.
For electronics work, I often check connections and then mark them off on my wiring diagram. I suggest doing this, it shouldn't be too bad.
Now final goofy Q: Have you connected the chassis ground lugs together? On mine there's a ground terminal on the chassis at the base. You may see some really weird behavior if the grounds are floating.
To be clear, are you off-grid right now or are you using a shore feed? Have you checked if your shore wiring is correct? This could be a goofy shore input issue causing odd crosstalk.