"1) How probable is it to find a teaching job at the Uni level? (I've got an MBA, TEFL Cert and some teaching exp)
2) Is Seoul the only center for this?
3) How do I choose a Recruiter?
4) Can I apply from another country if I'm a USA citizen?"
#1: It's probable, if you're in country and available for interviews.
#2: No, there are uni jobs all over the country and actually, it's not that easy to get a job in Seoul at a uni. You have to have some serious qualifications/experience in Korea/interview extremely well.
#3: Unis generally never use recruiters. They instead hire often through word or mouth, or on places like ESL Cafe. Any recruiter who claims they can get you a uni job is a liar and while you may get a job from them teaching uni students, the hours/pay/vacation will be like a hagwon.
#4: You can send in your application from anywhere. However, if you're not in country it can be next to impossible to get a uni job (I guess that maybe 95% of uni jobs in Korea require in-person interviews). And the other thing going against you is that it will be harder to obtain the documents you need for immigration, such as a Criminal Record Check.