Changed DMG password now it won't open
The password used for encrypting the DMG is not stored in a log file or similar. Having full admin access doesn't help you, I'm afraid.
The only real resort is to brute-force the passphrase for the DMG. It seems you have a pretty good idea of what the password is and how long it is - that should make brute-forcing worthwhile to try. I'm assuming here that you made a typo when setting the password or something similar to that.