CD3 and CD4 have the same active site and the same reaction mechanism, which is why you can cross process C41, ECN2, and E6.
Formalin scavenges any left over couplers, from what I've learned here. That is not specific to the developing agent. C41 films had had the same problem.
For CN films the manufacturers invested in development of a means to make the film stabilise itself without formalin.
You could do the same for E6, but it i would cost development money and it apparently didn't make enough money in the 90s to justify that.