I should note that you can do vector math of systems of charges, and for electric and magnetic fields, and have net 0 effects, but in the context of what i was trying to explain they don't really fit in, also in the ~4 hours i've been thinking about this i've learned a lot myself, just piecing things together, and i can think of a few things i need to read up on more,
basically the reason for equal and opposite voltage to the cell is to create equal and opposite stress on the oxygen and hydrogen, but they do not cancel out to zero voltage, since the term "zero voltage" would just mean "equilibrium of protons and electrons" while voltage is a unit of "difference" and can be measured between any two points regardless of their equilibrium of protons and electrons. If you have learned about "hho series cells" where they put 12 volts across 6 cells, for 2 volts between each cell, then you'll realize how "voltage" works as a unit of difference.
I also think that there is a bit more going on in Stan's injector, mainly, it screws into the engine block, aka grounded, which would stop you from getting the 20,000- volts, both positive and negative electrodes would have to be insulated from each other as well as the negative electrode insulated from the outside of the injector and thus the engine block. This, or one of them is grounded and 20kv is still enough to rip the water apart.
And on that note, ionized air (missing electrons) in the water fuel mixture can also absorb the milliamps of leaked current that gets past the chokes into the cell.