I realize this response is somewhat late, but I thought I would chime in anyway. I have a P18 and I have replaced the FD. Granted, it wasn't a 7970 FD or even close, but I had no issues at all. I am wondering if it has anything to do with the fact that it's a DI2, but that shouldn't be a problem. You have probably already checked this but, are you sure your "L" limit screw isn't preventing the derailleur from moving closer to the seat tube? What about the rotation angle screw?

On a side note, how are you going to route the DI2 power cables and how will that work when folding? Where/how are you going to mount the battery?

It sounds like this bike will be AMAZING when you're done. Please post pics!