Complex neutral earth bonding Quattro charge from main, invert, and backfeed

Q&A and troubleshooting
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I have a system comprised of two 3k 120v quattros setup in a split phase configuration. The goal of the system is to power some small loads as a UPS function, but then also in a large power outage be able to backed my main electrical panel in my house.

This creates some nuances with the MEN link that I’d like to ask about here.

There is a main house panel. This panel has a 50 amp breaker that feeds AC to the quattros so that the quattros can charge the battery. This AC panel has a neutral earth bond in the panel.

The output of the quattros goes to a small electrical panel. This panel feeds a few outlets but also has a 50 amp breaker that is destined to back feed the main panel. This panel has no neutral ground bond.

In order to avoid the case where the Quattro could be taking power from the main panel and back feeding it to itself the AC feed from the main panel, and the backfeed feed to the main panel are both connected to interlocked contractors with an on-off-on switch controlling them. In this way it is not possible for the Quattro to backfeed itself. In both contactors, L1 L2 and N are broken.

It appears as though I have 3 cases.

  1. Quattro taking power from main panel and charging batteries. In this instance I want the ground neutral relay in the Quattro OPEN as the link is made in the main panel
  2. Island mode, both contractors open, no backfeed and no AC input. In this case I need the neutral ground relay closed.
  3. Backfeeding the main panel. In this case I want the neutral ground link open as the link is made in the main panel.

So effectively I have a situation where the presence of AC in is not sufficient to drive the state of the internal ground relay.

What I think I can do is this. In the sub panel, add a contactor that bonds the neutral bar to the ground bar. Then use the aux relays on the ACIN contactor and the BACKFEED contractor to open and close the neutral ground link.

In situation 1. ACIN contactor energized, aux relay keeps GND neutral contactor open. GND neutral link made in main panel.

In situation 2. Both contactors open, GND neutral contactor closed. GND neutral link made in sub panel.

In situation 3. Backfeed contactor is closed aux relay keeps GND neutral contactor open. GND neutral link made in main panel.

In doing this I would disable entirely the internal GND neutral relays in the Quattro.

Does this make sense?