I was in your situation about 12 years ago, joined the Navy, was lucky enough to fly super hornets and have no regrets.
1. I had a b.s. in Geography with a 2.9 gpa.
2. I started OCS at 26 years old, I believe you have to be commisioned by 27.
3. I admire you for wanting to serve, but if you have airline aspirations there is no job other than Pilot to get you there. I think the only other "cool" Navy job is SEAL.
4. Giving up 4-6 years will delay your airline career by at least 4-6 years.
5. has been answered.
Keep in mind that you are commiting to about 11 years min in the navy and there is always a chance you could end up flying helicopters (not that theres anything wrong with that). The great thing about OCS is that you are guaranteed a pilot slot. All you have to do is make it throught OCS (not too hard), API (not too hard) and get jet grades in primary (a little harder).
I agree with Spanky that the Guard is also a good option.
Good luck.