Yes, but those are so competitive that with a 3.6 HS GPA, they are likely to be unrealistic reaches.
Not unless the specific colleges you adjunct at give any kind of benefits that may be relevant (but adjunct faculty get few or no benefits generally).
Yes, but you need to check each college specifically and for the specific majors of interest. This is assuming that you mean capacity limitations, which are the typical administrative barriers to changing majors. Regardless of whether there are any capacity limitations, a student who is undecided or is considering changing major does need to take courses to stay on track in the possible new major in order to avoid needing extra semesters.