Melia a Mary-sue??? Huuuh..Are we playing different games with the same character named Melia?
Melia has plenty of flaws, flaws that come back to bite her, she isn't perfect, she is always conflicted, always tense (even in the slow moments), Tesla even let her go to the Spa for free and tells the MC that it's pretty clear she's been through a lot, in one of the chapters she even has to deal with the problems of her "I want to help everybody!" mentality, she isn't untouchable and a lot of the times she is powerless to deal with a conflict (which by Mary Sue definition, should be the opposite), one of the very first conflicts she has is the fact that her life is a lie, her father isn't actually her father and she doesn't know how to cope with that, in one hand he assures her that he loves her enough to defy orders in another he still lied to her and is working with Team Xen and the whole reason he even played the father figure was to keep a eye on her until she "becomes ready", compare Melia with a character like Captain Marvel, which is a massive Mary-Sue, and you'll clearly see they are nothing alike..
For the part that she says she steals the MC's credit I never experienced that.. In fact everybody credits you too much, always looking at the MC as the person who ends all conflicts, now that I think of it.. The MC is more of a Mary-Sue than Melia..
Anyways I would tell you to keep going.