Congratulations on getting your HT where you want it and I am in the process of getting some questions up for my own dilemma, but your situation also puzzled me: 1) I had no idea that in-ceiling speakers could measure so low and I didn't see where they are rated at 20 Hz on the bottom end 2) I don't understand why the crossover on them and your surrounds would be lower than your mains.
I have 4 in-ceiling speakers (mid-range Speakercraft) and my Integra DHC-80.3 consistently crosses them at 40Hz post XT32 calibration. I only have a legacy setup for REW measurements and haven't bothered to manipulate the connections on the back of my prepro to take some measurements of these speakers but I also find it very peculiar. I usually just raise them post calibration. I think some have speculated it could have something to do with how they are mounted in the ceiling?!