the fact that gs.addErrorMesasge shows my error messages, indicates to me that gs.flushMessages() should also be able to clear whether it is in the Action Assignment or Script Include (in my case, both)

I'll have a play, but it is more the stringing together / timing that I'd need to address.

Ideally it would be clear message, process, show message or continue.

It needs to be a Server Script as it is working on flow data and can only happen in the action assignment server script otherwise it allows the platform to continue and the intention is to stop where it is if there is an error - aka standard validation on a standard field/variable.