Sounds like the fan might be standard at least. I'm guessing they flip flop on how they put everything together so that businesses never have the right parts in stock and have to get them from Dell.
With the front panel header, I suggest ohming it out with a multimeter. That means pulling the pins from the new chassis's header and plugging them in individually. Should be fairly easy to find things like the activity lights.
I have found wiring diagrams online a few times, but I guess the question is how important is the front panel to you? The recent 9020 I scavenged a motherboard from we just flipped the computer around and used the rear I/O for everything. And as you said the USB 3.0 connector is standard, so we were able to get...