I think this is a good idea. I checked and the Growatt has continuity between AC In Gnd, AC Out Gnd and Inverter case, so we only need to connect one of the inverters Gnd terminals.
second contactor from main to GW inverter
I believe you don't need this second contactor from main to GW inverter, it is just redundant to what is already build into the GW. The inverter has AC IN voltage sensing and will disconnect the internal contacts when the voltage is no longer good.
It will only disconnect L1 and L2, Gnd remains connected so that we do not have to add a second ground rod.
Switching N/G Bond with GW dry contacts
- I think this is a good idea, this prevents the AT from being paralleled to Grid Tansformer.
Use a 120V coil, you need to get power from a service panel, run it through the dry contacts then to the coil of the contactor.
N/G Bond
I had told you that you cannot have two N/G bonds at the same time, but that only applies to Inverter ON-Grid mode.
When Inverter is OFF-Grid the power source originates at the inverter, the original N/G Bond in the main service panel is no longer effective 'it is out of the curcuit'. So we need to create N/G Bond with the Contactor controlled by the GW dry contact. the two N/G bonds at the same time is fine. Being that the internal contacts in the GW have L1 and L2 disconnected, this puts the two N/G Bonds on completely different curcuits.
Lost Neutral Monitoring
I think it is fine monitoring only one leg if you monitor both Over Voltage and Under Voltage.