you will not get a working visa without a valid degree, that is for sure.
the average "starter" wage stays the same irrespective of your experience, you will not get pay rises but teaching experience in Japan does open doors to more lucrative work, such as teaching business English.
The pay is not a salary, it is a monthly wage, and you will not be paid for the summer months or only in part as a retainer, you also may receive a retainer for the winter break.
I have have had over ten years experience teaching English in Tokyo and Yokohama, and am sure other people with similar years exp will attest to this.
Also, under Japanese labor law, you cannot be hired if you are under the age of 20 as you are not legally an adult in Japan and as such would not be covered by workplace insurance, nor would you be covered by the dispatch requirements for workers (i.e: being sent offsite to teach, to an elementary school for example).
I have no idea about repaying a student loan as i am not a money lender.
Rent can be as little as 30k a month if you live in shared accommodation in what is known as a gaijin house. Hope this helps you.