tgb5545 I don't necessarily trust that since they had to look up what an operating system was before giving me an answer.
I wouldn't worry about that. Customer service reps aren't usually tech experts (or even basically knowledgeable). They just follow a script.
Mint is a reputable company, and if they told you they would unlock it, it's likely they will.
tgb5545 Is there a workaround or do I need to return the phone?
There is no workaround, other than paying off the phone, or possibly waiting 60 days.
tgb5545 Is there any OS I can install?
Nope. You're stuck on stock until you can unlock.
Edit: however 23Sha-ger makes a good point about what "unlock" means here