i didnt exactly follow and i'm not 100% sure its relevant, but FYI the DX7 PPM is troublesome indeed. it's supposed to be JR compatible afaik, but its not always working.
for example flytron's usbtoppm doesn't work with my dx7 but works fine with all my other non spektrum remotes, while the PPM signal is quite similar.
here's some audio screenshot of the ppm stream i was using instead (mostly reversed from looking at the dx7's one over audio):
i've to look at the code which generates this again but i think this was the correct screenshot
at some point i changed the frame length as it wasn't the one i expected.. might have been 22.0.. but don't have the code here.