The usual solution with a "standard" boat is two reversible motors, each with its own ESC, connected via a mixer. Two ESCs, plus one two channel mixer.
If the bilge pumps are non-reversible, pair up a forward and reverse, and feed each one via a rectifier diode. That way, the forward motors will respond to a forward signal via its diode, the reverse one to a reverse signal via the other diode. (Assuming brush type DC motors in the pumps).
I am not familiar with the mixer you mention, so I might well be making some incorrect assumptions, but the installation does seem to have become overly complex, and needs to be got back to something simpler.