That folder is indeed created because you entered the wrong JPS password.
No, we cannot (and should not) check the encryption key beforehand. Remember that if there is a quick and painless way to check whether the encryption key is correct then the encryption implementation is susceptible to very fast brute force attacks. That's pretty much why the very old password-protected Excel and Word files could be "cracked" relatively easily even with the relatively feeble computers available 20 years ago.
JPS does not have any data which can be used for encryption key verification. Its layout and the ideas behind it are explained in https://www.akeebabackup.com/documentation/akeeba-backup-documentation/jps-archive-format.html
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!