My only other thought was to go way back to something like 2.3.0, as I suggested above.
Just was worried what it might do to the hub database. But you are correct you could take and save a backup prior, do a soft reset, try the firmware update, then restore the backup after you have updated the platform again. I will look in the release notes and see when they published the 7.15 update.
2.2.3 had a C7 firmware update (I think this would have been the 7.15 update which you need): Release Notes: 2.2.3 | Hubitat Documentation OR Release 2.2.3 Available You could try getting that platform.
Found where they added 7.18.1 Z-Wave FW: Release 2.3.4 Available
Also be sure not to download too many older platforms at once, always want to make sure a current one stays in the diag tool list in case you get stuck you can get back to current.
