Hi there -- I'll try to help with your questions...
Regarding #1. "Do you think Portland State University is legitimate?"
-->I can't say difinitively, but but gut says probably yes. I'd research with your university and outside bodies to be positive.
2. Is the T.E.S.L just as recognized for overseas teaching as the C.E.L.T.A and the T.E.F.L because my goal is overseas teaching?
--> This shouldn't be a problem. I've researched this question, and both are recognized. Sometimes the CELTA is preferred in the E.U., but TEFL is likewise recognized.
3. I'm a native speaker of English, and I want to be the best teacher I can be, but I get a bit worried when I think of all the English I don't know-- grammar, structure and participles.
-->Depending on the age of students, course you teach, etc., these aspects of grammar and structure may or may not be important in your job. Generally, as a native English speaker and with the education you're embarking upon, you'll be able to handle the majority of grammar questions that arise. If somethings comes up you don't know 100% on the spot, you can always research it and give the answer during the next class. I would keep trying to learn as much as possible, but don't stress unnecessarily about being prepared.
4. I know that after four or possibly five years of college, I will have built up student loans ...
--> This is also an aspect of teaching I've contemplated, researched, (and face personally). Perhaps the best way to pay back loans is to first teach in an Asian country such as S. Korea or Japan where you can earn enough to pay back loans. Also, you can save up enough to then go to Brazil, etc. where the pay is less.
Hope this info is of some help to you! Best of luck with your future plans and current studies!