I assume you are booting from the Recovery Drive partition on what was the Fusion boot drive? If so you might try booting from the Internet Recovery Drive. To boot from the Internet Recovery Drive power down completely, then boot while holding down Command+Option+R (it is Option key that makes the difference). You will first be prompted to logon to a network for internet access and then you will eventually be presented with essentially the Recovery Drive menu you would get booting from the Recovery Drive on your boot drive.

You will, of course, have access to a full version of disk utility and should be able to perform the same operations you were attempting, but there should be nothing open on either the HD or SSD portions of your Fusion drive. I don’t know this will work for sure, but it is certainly worth trying. Good luck 🍀

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein