High Sierra is a mess, I had someone whose computer was un-bootable because of it. I went through many "solutions" only to finally roll it back to a previous OS version via backup (thankfully they used Time Machine regularly), and told them do the current system updates but don't update the OS.

I've worked through many update issues in OSX over the years, and the fixes were reasonable, but this one is really bad, and sometimes there isn't any workable solution than to roll-back.

I've noticed the OSX/iOS upgrade push is really affecting a lot of people in different ways lately, another has iPad/iPod issues because you have to update OS to get a never version of iTunes to talk to upgraded iOS devices that are now locked out of previously compatible versions of iTunes - and with device signing theres no way to roll back the mobile devices iOS... and with Mac App store theres no way to get an update thats not High Sierra anymore so Macs that can do say Yosemite but not HS are S.O.L. (unless you already have the install media) Blegh.