You won’t stop ACIn > ACOut with Charge Only. You could switch off ACIn while you’re inverting to the remote.
Not sure what you mean by “remote”
If that is the case what is the purpose of “charge only” mode?
In any case, I already have an external relay that takes care of that by sensing when the inverter output goes down 
but that won’t work if, as you said, the AC Out will be on even in Charge Only mode.
That is already happening but I was under the understanding that in charge mode only there would be no mains out at all, not from the inverter nor from the ACin.
Currently, my external contactor relay stops AC-out to the services if the multi stops inverting without allowing any of the ACIn to pass through.
I see what you mean, I never noticed the last sentence 
When the switch is switched to ‘II / Charger Only’, only the battery charger of the Multi will operate (if mains voltage is present). In this mode input voltage also is switched through to the ‘AC out’ terminal.<<
In light of the AC Out being always on in the presence of ACin, yours seems the only possible solution, I’d have to swap AC-Out 1 for AC-Out 2.
I have already created the necessary assistants for the job dependent on the ACin presence.
I can then use the signal that externally connects ACIn to also control the grounding conductor relay.