Major and minor

No. As long as the 30 credits are from the “2000- and 3000- level” category, you meet the requirement. In other words, all five courses could be 2000-level courses.

Yes, you can, as long as you meet the prerequisite requirements and that the course is not full, you are eligible to enroll. Nevertheless, 4000- level courses are mainly seminar [...]

Yes, you may change your declared major(s)/minor(s) during the course selection period from the third to seventh semester.

You will have to declare at least one major at the beginning of the second year of study.