My tought on water leveling is more complex than yours but can prevent cell shorting.

Once the water level reach the set threshold, the floater seals the filler hole. In practical, this design requires precision because of 3mm cell compartment size.
On anti slosh method, I was thinking of total seal the top of cell compartment but insert two long 1" x 3mm hollow tube far apart from each other. This could prevent the water from pouring out because of narrow & higher hole but will allow gas to flow through.
Bob, the plate is not really clean. There are some oil film, thumb prints, dirts, glue, etc on the plates. I have to use acetone to clean the plate surface then rinse with distrilled water (already did that). I won't soak the plate into acetone. For conditioning, I will use the same method as before.
I have assembled one cell compartment yesterday. So far no water leakage. 6 more to go. I going to post the pictures soon.