I would get as much liquid as you could out of the zucchini after shredding it and then go with coconut flour (maybe less then the recipe called for - I'd try 1/2 first) and butter.
We made salmon cakes the other day - sort of similar recipe - 2 eggs, some chopped onions and stuff and for the 14oz can of salmon only had to use about 1.5 tbsp of coconut flour. They didn't stay together quite as well as a "real" salmon cakes but they stayed together enough to flip and keep their patty form.
What I did which I think may have helped was to mix everything together and then let it sit in a bowl, covered with a paper towel, in the fridge all day - I thought maybe this would help get some of the moisture out and let the coconut flour absorb the liquid better... Don't know if it did or not.
My husband hates all things coconut and liked the salmon cakes - so if you are afraid of tasting the coconut, I think in small amounts it goes undetected.
Your recipe sounds good though - I think I may try them!