Still with no Translucence enabled, looking once more from behind, with front lighting, we see translucence. !!? Why is that?

The Translucence channel has only one job - to convey BACK lighting to the front. It is not responsible for conveying FRONT lighting to the back. As we can see this is already performed by the completely ordinary lighting of the built-in diffuse node (or any diffuse node).

What does this mean? It means that, for example, if you're looking up into a tree with the sun shining down, and the "front" of the leaves face up, you're looking at front-lit back-viewed paper-thin, and that is NOT the job of translucence. That will work no matter what.

You ONLY need translucence when you are BACK-lit front-viewed paper-thin. Change any of those three factors and it will not help you at all.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)