How ethical is zabla where each side chooses a dayan whom may then feel partial to the litigant that chooses them & may advocate for them?
This is a valid point to raise, and experience shows that this is often a problem – the selected Dayanim prefer the litigant who selected them, so that the Zabla effectively becomes a case of Dan Yachid, with the third Borer being the one to decide the case.
See Sema (7:20) who mentions that this is the basic mechanism of Zabla.
For this reason, there is room to prefer a regular Beis Din setting than Zabla. And yet, sometimes Zabla is the easiest option, since both sides rely on “their Dayan” to convince the third Borer in the truth of their claim.