Weeks before I posted my original query, I e-mailed the tech desk at Whale pumps with the same question.
Now, after having turned my cockpit floor into an occasional foot bath, I got a reply:
"Our pumps have a non return valve inside the outlet port
so you should be fine without a backflow preventer valve."
So I could'a used the Y connector and the original transom bilgepump through-hull after all.