Good news! I think I found a solution:
(1) Boot the Mac from any volume containing an APFS-aware version of macOS (that is, Sierra or High Sierra) other than the subject volume (the one you're trying to remove the encryption from). It doesn't matter whether or not the volume you boot from is itself formatted in APFS.
(2) Log in if necessary, and make your way to the Terminal. (If you've booted using a Recovery volume or a macOS installer volume, you can find Terminal in the "Utilities" menu.)
(3) In Terminal, enter the following command: diskutil apfs list
(4) In the displayed results, look for the name of the subject volume, and make note of its disk identifier string, which should be in the format disk#s# (where "#" represents a numeral).
(5) In Terminal, enter the following command: diskutil apfs unlockVolume disk#s# (substituting the disk identifier string for the subject volume). When prompted, type in the volume's "Disk Password" (encryption key).
(6) In Terminal, enter the following command: diskutil apfs decryptVolume disk#s# (substituting the disk identifier string for the subject volume). When prompted, type in the volume's "Disk Password" (encryption key).
If all is well, you will see a message indicating that decryption of the subject volume has begun. If so, it's just a matter of waiting for the process to complete, which may take several hours. If you wish, you can immediately restart the Mac and boot up the subject volume. You will be asked once more to provide the Disk Password before you can log in. After logging in, open System Preferences > Security & Privacy > FileVault. You should see the label "Decrypting..." next to a progress bar with an estimated time remaining to complete the decryption task. Once it's completed, you should be able to re-enable FileVault from the same prefpane, and *this* time your admin-privileged account will be able to administrate FileVault properly, including the ability to log in at startup without separately providing the Disk Password first.
Give this a try and let me know how it goes for you!