Re-Theme, Re-Implement - Legal Issues?

Quite a few mechanical changes soulds to me as it is different engouh to be on the save side regardin copyright issues.
So if thats the case - and I want to trust you without reading the rules of both games), I´d conclude it is legal to publish Zomie Blitz.

The other question is, how do you feel about the game. Is it yours? Or is it your variation of Werner Ernst George Muller´s game? And try to put yourselfe in his shoes. Would you feel betrayed? Maybe the best way to find out is to try to contact him and get his OK.

I don´t think we should only look at whats legal. The law is verry weak regarding copyright of boardgames mechanics / systems etc. So the only way to protect our ideas is an moral commitment of game desiners, not to vary games of other designers to bypass copyright.
But maybe your game is much more different, so this does not apply in this case.

An examples that come to my mind is the Russian version of Hey, thats my Fish" with a shark.
Forther, there is Dobble. It´s almost the same as Kunterbunt (Catch the Match) with a new artwork and less pictures per card.
Or look at Mutant Meeples. Is it more than a re-themed Ricochet Robots with a little variation?

Dutch Blitz was printed in 1960, so even if it had been patented, the patent would be expired by now. On that count, I think you're ok. I would be inclined to change the "Blitz" part as well, because as mentioned above, that may poke over into trademark issues.

In a less specifically legal sense (the "moral self policing" end), I'd say given the age of the game, you shouldn't worry about stepping on toes. 50 years is more than enough time for someone to see the fruits of their labor, and Dutch Blitz seems to share more similarities to Nertz (printed in 1930) than yours does to his, so if you wanted to go the further step of asking permission, I'd ask the designer of Nertz. Not only is he probably not alive, no-one is credited as the designer, so I'd say you've done due diligence.

My understanding was that they had got the game IP anticipating that it would make getting the Book IP easier (sense of continuity for the Estate) and would have absolved issues if they had done it the other (I.e. if they had got book IP first they certainly would need game IP)

And the elephant in the room, the game IP was a lot cheaper than the book IP.

Because when they did so, they had an expectation of getting the book IP as well. Getting the book and not the game license would have meant not being able to rely on any prewriiten material.

Plus, while FFG is the 700lb. Gorilla of US game companies, there is a moral issue which I think they wanted to stay on the good side of the many fans of the original game.