Since measuring I've gotten to work on the crossover with a ton of help from tuxedocivic of the AVS forum, who kindly offered to gate the frequency response files and determine the acoustic offset to get me started. When determining the acoustic offset, we found that it was possible to match the shape of the curve from the parallel driver measurement, but that it was a couple dBs down from the summed response in PCD due to the individual driver measurements. Tuxedocivic suggested that it might be because the amp wasn't able to output enough current into the low impedance from the drivers in parallel.
Here's what the offset test looks like (the grey is the actual measured summed response, and the black is PCD's summed response based on the individual measurements):
To test that theory, tuxedocivic and I put in Bwaslo's crossover design as a reference point to see what was going on. With the z offset entered in PCD, there was a huge dip right around the crossover frequency that we knew wasn't from the crossover design, so tuxedocivic removed the offset, and the response was flat like it should be. (Just as a note, my measurements aren't the greatest, so I don't think it properly represents Bwaslo's crossover design. This was just used as a test to see if the offset was usable.)
Bwaslo's crossover with .06 z offset:
So that leaves the question of whether or not that dip actually exists in the system, or if it might be a result of not having enough amplifier for the measurements. Without knowing the answer, however, I just went ahead and played around in PCD to make a couple crossovers-- one taking the z offset into account, and one not.
Here are the results of my first crack at it:
Accounting for .06m z offset:
After showing these to tuxedocivic he pointed out that the one accounting for the acoustic offset definitely isn't a good option. Since the summed response is lower than the tweeter level near the crossover, just going slightly off axis can remove the phase cancellation in that area, and will result in a big ugly peak where it used to be flat. The one without the z offset, however, looks like it could work out, and it's actually a much simpler (and cheaper) crossover.
I've still got a lot of learning to do when it comes to crossover design, but I'll be putting together a mockup of the "no z offset" crossover to test. If everything goes right, and the z offset isn't actually that pronounced, it should look pretty good, and I'll probably go ahead with that design.
(note: all of these plots were created in Passive Crossover Designer 7, and the response below 250Hz is meaningless due to the gating used)