I'll try to answer a few of your questions as a Marine flyer (KC-130s). I also joined as an air contract through PLC, so I can try and give you a little bit of an insight into that (though it was 7 years ago). Foremost, you are a Marine Officer first; your primary MOS comes second. What this means is that you are in a club centered around the infantry from the moment you show up at the OCS parade deck. If you are only interested in flying an airplane, or if you advertise your intense interest in aviation at OCS, you are not going to do well. We had one guy in my PLC platoon with a bunch of civilian hours and quals get bounced in week 5; he had a few problems, but supposedly he told the retention board that his goal was 'to be the best pilot the Marine Corps ever saw', and was shown the door pretty quick after that. I munched the ground side stuff up, but still got my share of ribbing from sergeant instructors, candidates, and (later at TBS) other officers for being an air contract. On that note, should you graduate and commission you will then head to The Basic School for six months, where you will learn to lead a provisional rifle platoon. While not the sustained screaming/job interview that is OCS, it is still pretty intense field training that is going to have you feeling a lot closer to the infantry than wings of gold.
After you get through all that, you head off to Pensacola to start flight school. For me, with the training, plus a slowdown in the pilot pipeline, it was almost exactly two years from when I commissioned to when I started API (Marines have only a few slots in each class of student Naval aviators, the rest are Navy and Coast Guard, therefore it tends to back up the flow of students coming in). In the meantime, there's a lot of ground training and TAD's to the ground units at Lejeune to be had.
As for flight school, ///AMG told it pretty well with the NSS system (this is the grading system the Navy uses). There's no "1 percent" sort of deal going on, but jet grades were just above 50 four years ago when I went through. Again, like ///AMG said it can all come down to slots available, the other guys' grades/preferences the week you select in primary, and your NSS and that bell curve. I knew guys with really high NSS who didn't get jets, and other guys with 50+ NSS who got jets and didn't want them. There's also a minimum NSS to maintain, and it's normally higher for Marines than it was for Navy guys. If you fall below it you get dropped, lat move to a ground MOS, so again, hope you liked TBS...
I would say it doesn't matter a whole lot what you study in college initially. My bachelor's is in International Politics, I hate math, I knew very little about how airplanes worked before, but I had an NSS competitive for jets. Another Marine I went through most of primary with was an Embry-Riddle grad with a ton of civilian hours; I'd say it gave him a leg up early in contacts, but by the end we were pretty much even. The main thing is just having the ethic and drive to study and put the work in once you get there. They'll teach you everything.
As for Marine aviation: if your true goal is nothing more than to fly a pointy, shiny fast thing doing the latest and greatest air combat maneuvers, really talk to the Air Force, Navy, and Guard bubbas here. Marine aviation is centered around supporting the ground Marine; lots of close air support, assault support, etc. Also, the Marines are traditionally at the short end of the budget and supply chain. The Prowlers and Harriers are super old, the -18s are pretty old too. I fly J-model Hercs, which along with the Osprey are some of the newer aircraft in the inventory, and even we are worn thin and often hurting for parts. That said, we are heavily utilized, the CH-53K will be out in a few years, the UH-1Y and AH-1Z are on line, and if the F-35 flies right and doesn't get attacked by Congress, there's a pretty good forecast for Marine aviation. You can fly a fairly new plane and get a ton of flight time depending where you're at. And most guys I know love what they fly anyway (I wanted Hercs as my first choice, and have never looked back).
Like I said, talk to the other services too, and feel out what it is you want in being a military aviator. If you have any more questions feel free to PM me.