We just purchased a new convertible car seat for my son who outgrew his infant seat, and we ruled out Diono specifically for this reason. They are well regarded seats, but I think headrest interference is going to be a problem with just about any new car. Despite what the Pacifica owner's manual says, I think that installing the car seat with the headrest down is a last resort option. I tried that just to see if it worked and I noticed two downsides. First, I could not get the car seat to the proper recline angle without reclining the vehicle seat quite a ways back. Second, it was too easy to move the car seat left and right, it wasn't stable and secure enough for me.
Our choice came down to the Britax Boulevard and the Chicco NextFit. Both of these seats fit forward facing in the 2nd and 3rd row, with the backs of the car seats snugly against the vehicle seat back. We didn't have any problems with the headrests pushing the car seat away from the vehicle seat back, although we did raise the headrest by a notch in the second row. So my recommendation for anyone else who finds this thread is just buy a seat that fits.
I also don't understand why Chrysler located the latch anchors in between the left and center positions of the 3rd row. It's an awkward place. I would not use that position unless you really need to use the latch anchors for quick installation and removal of the car seat. Instead, I use seat belt installation with the car seat in the left seat.