Cannot upgrade to High Sierra: Unable to unmount volume for repair?
Unfortunately, the solution to my problem was to wipe and do a fresh install of High Sierra and then restore from my Time Machine backup on first boot. Here are the steps I took. You'll need the following to do this:


An external USB thumb drive with at least 8 GB of free space
An external hard drive with at least 50 GB of free space


Make sure you have a functional, up-to-date Time Machine backup

To do this I did a manual Time Machine backup than deleted and restored a file I care about.

Download High Sierra and make and an installation drive

Download the High Sierra installer from the App Store. Attach the USB thumb drive to your Mac and then make a bootable, High Sierra installation disk out of thumb drive by running the following in Terminal:

sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ High\ Sierra.app


Where /Volumes/Untitled should be replaced with the name of your USB thumb drive.

Format an external drive and install High Sierra on that drive

I used an older 500GB LaCie I keep kicking around for just these cases. It's fast and built well so I can trust it. Use the attached thumb drive to run the High Sierra installer. When asked which drive to install the operating system on to, click the Show More Devices button and select that USB hard drive. The installation should proceed without a problem and when the computer reboots you'll be booted from the remote drive and dropped into High Sierra. You can skip the first boot setup. You won't be using this for very long. Just make a temp account and get to logging in.

Force unmount the internal fusion drive and format it

Here is where the scary bit begins. You're going to force unmount the fusion drive because it'll still be mounted (and stuck) when you boot the external High Sierra image. To do this I opened up Terminal and ran:

sudo diskutil unmount force /Volumes/Macintosh\ HD


You should replace /Volumes/Macintosh\ HD with the name of your fusion drive before you run that command. Once the drive is unmounted you can format it like so:

sudo diskutil reformat /dev/disk2


You should replace /dev/disk2 in the above command with whatever your disk number is for your fusion drive.

Install High Sierra to your fusion drive using the installer

With the drive wiped you can proceed to install High Sierra on it using the USB thumb drive you turned into a bootable, High Sierra installation drive. Same as before, run the High Sierra installer from the thumb drive and when it asks you which disk to target select Show More Devices and find your fusion drive in the list.

The installation should proceed without a problem now and when your machine reboots you'll be booted from the internal drive.

Follow the first boot prompts and restore from your Time Machine backup

You'll have to pick your region, connect your keyboard and mouse, and then setup WiFi. Once that is done the first boot process will give you the option to restore the computer from a Time Machine backup. Select this option. Select everything on the backup that you want restored and follow the prompts.

Once the restore is complete you should have a functioning High Sierra installation with all your data and user accounts on it.