I’ve run out of time this evening to finish reviewing your patch, unfortunately. Your code looks ok, on the whole, but what this has reminded me of is what an awful mess the existing grid code is generally. We had a discussion about this a few months ago:
https://forum.vassalengine.org/t/adc2-to-vassal-coversion/2635/1
Adding more grid code without refactoring sends us further down the rabbit hole. I think a large part of the difficulties you had in extending HexGridNumbering stem from it’s kitchen-sink design. This is one of those places where it would be relatively easy to disentangle all of the parts so that everything is coded to an interface and each class does one thing, and also write unit tests for them. Would you be interested in doing this if I helped you get started?