I did "strings" on "system_diagnostics" and found out what causes the "device_info.xml not found..." message when you try to disable diags from the menu.
You can create the missing file with:
mkdir -p /mnt/us/diagnostic_logs
touch /mnt/us/diagnostic_logs/device_info.xml
I first renamed my diagnostic_logs folder, then did the above commands, and then the diags menu successfully disabled diags and rebooted to main.
Those two commands should be added to the jailbreak instructions between steps 12 and 13.
The "disable diagnostics" menu adds an xml stanza to the xml file that identifies the diagnostics version number. Other diagnostic tests add other identifying info to that file as well when you run them (but they are smart enough to create a missing folder and files).
EDIT: For those who did not understand the purpose of these linux commands, they go in the RUNME.sh file, or better yet, the payload dropped by data.tar.gz. They do nothing if the folder and file do not exist, but they create them if they are missing. All automatic, with no manual steps or worry by the end user. The USER does not need extra steps. Those are part of the payload...
