Anyway... have you tried using the openssl method to decrypt the .key file?
openssl enc -d -aes-256-cbc -p -md md5 -a -in multibit.key -out newtest.txt
This command (on Linux, but you can also install openssl for Windows) will prompt you for the password and then decrypt the .key file (simply change the multibit.key part to match your .key file) and create the .txt file that contains the decrypted text:
This is my "key_test.key" file:
U2FsdGVkX18u5HQe1dGcYI5vGqObV/G/+nHAoafGmGhrcAwz40smBfsj/B+VurCwtAC0Ba9QMoEU
lB2dMrGBLdDyYxtiQ1GsyYg3CZkSDZZmvlVo7pnYIO3fGCTmc85d
This is what happens when I run the openssl command in linux:
This is the contents of newtest.txt:
L2p3VjkRXfwAY8kfemFsh8HJ6Pfn4DxLxxsdL8XvZVfnTDZkGLjN 2021-03-30T17:17:06Z
If you're getting garbage output, it sounds like the .key file might be corrupt