Gene has put considerable effort into trying to id and correct this issue. I have helped with some of the testing. The results seem to confirm the conclusion that the PiX10 hub is not compatible with the cm19a. It seems to be a systems issue, ie HW + SW, as the cm19a does seem to play well with the Pi Zero w in one known instance where it is used as an Alexa Hub. Or that could be a "Random Success".
Yes in all the time I've been involved with HG I've never witnessed Gene put so much effort into trying to resolve one or two users problems with what would be termed a minority technology such as X10.
While you state that your conclusion is the Pi X10 hub is not compatible with the CM19 driver that is not my experience and indeed Gene is of the belief that it is working too. It's unfortunate that there were not a few other users on the Github to confirm or deny your conclusion.
I did suggest that you either split the load on the Pi Zero and shift one of the servers to a second Raspberry Pi or purchase the more powerful Raspberry Pi 3+ as the platform for your Pi X10 hub.
The important point here is the Pi X10 hub is the software and the Pi Zero was the platform it was chosen to run it on. The software is fine. Should you wish to continue running the Pi X10 hub I do suggest you purchase the Raspberry Pi 3+ to enjoy the full benefits of the HG/HA Bridge combination and in your situation Alexa.