Is the Encryption Key for Adoptable SD card included in manual backup done over MTP?
Unfortunately, what you have so far considered as the complete copy of Internal Storage is not true. You only copied (I suppose using MTP - "transfer files" option) your files which you had created or downloaded in that storage since last factory reset. The app and system specific private files are never copied in this fashion. /data/misc/vold cannot be copied directly using MTP. A user needs root access to access that file (includes copying it).

In conclusion, you have hit a dead end on this part.