Test password variations; as you mentioned a possible issue with UTF-16, try removing or adding trailing spaces, test versions with and without uppercase letters, and if your password contains special characters, convert them to their ASCII forms
Perform brute force; extract the base64 data from the "encryptedPassword" and use a script to test various password combinations. If you're able to decrypt this block, then that's the correct password
You can also perform brute force directly on the ".wallet" file. Since you have "continuous backups" and ZIP backups, you have multiple snapshots to test — sometimes, an older file may not have the same corruption as a later version

That's all I know so far; I hope I can help you