Sounds like it should be pretty simple to show the dealership. Look, no diode and it charges the new shiny leaf on your lot. No diode and my car doesn't work. Add a diode, it now works on both. One of these things is not like the other, fix the OBC Nissan...
Ha! You're confusing this with a customer service experience that cares! My car charges - on basically ONE EVSE. The one at the dealer shop. And it charges on the little110V device that came with my car. Because it charges on the two Nissan devices (that fail to check for that diode), the dealer (and the company by the way!) claim there's nothing wrong with the car, only with the untold millions of installed EVSEs in the world that actually conform to the J1772 communication standard.
They are, of course, willing to replace my OBC at my expense if I really want to be compatible with non-Nissan-branded EVSEs.