Can I roll back from Mojave back to High Sierra?

You can if you have either a full Time Machine backup or a bootable clone backup. In either case, you will need to erase your drive completely in order to reinstall the earlier system. If you do not have a backup, you will not be able to.

I am not sure if Time Machine allows this but Carbon Copy Cloner will allow you to restore a previous macOS, IF you have backed it up.

For what it is worth I tried out my critical software on a second Mac before making the leap to Mojave on my main Mac. I was luckier than you.

Note that the next macOS will not run 32-bit apps, which means Mojave may be my last macOS for a long time!

I’m sure you meant to address the OP?

I suggested Time Machine because it can:

https://support.apple.com/en-us/HT203981

I have bootable clones of every OS since Mavericks (and a Mac to run them on), all done with CCC.

FWIW, I installed Mojave on my MBA and haven’t had any problems.