You would want to get a copy of your BA diploma apostilled. To get your diploma apostilled, I will provide you a link for more info: https://www.korvia.com/apostille-korvia-guide/
Yes, you will have to submit a letter of intent to graduate, however, you have to make sure that you have enough time to have your diploma apostilled and have it sent it to Korea.
Yes, that will substitute your MA degree and transcripts until you receive those. But, you must make sure to obtain them and have your MA diploma apostilled before coming to Korea. (if your MA is education related, if not, you don't need to have your ...
Your letter of intent to graduate does not need apostille :) you only need to apostille the BA diploma after the graduation (and background check too of course)