Can't decrypt adopted storage
Good news: trying the following command successfully decrypted my SD card without any errors:
sudo dmsetup create crypt1 --table &quot;0 `sudo blockdev --getsize /dev/mmcblk0p2` crypt aes-cbc-essiv:sha256 [16-BYTE HEX KEY] 0 /dev/mmcblk0p2 0&quot;

A couple of minor issues:

sudo mount -t /dev/mmcblk0p2/crypt1 /media/mint/sd returned the error mount: /media/mint/sd: can't find in /etc/fstab. I can view the content of the SD card in the file browser running as admin, however.
Copying the files over to my laptop's Windows 10 SSD, some files and directories didn't get copied due to the presence of invalid characters in their names (mainly asterisks and colons in file names and a period in a folder name). Not sure how to deal with these...

Thanks for all your help.