When should I contact the selection officer?
As soon as possible. The sooner that you can secure an aviation slot for yourself, the better. The process is a bit longer to become a SNA/SNFO (Student Naval Aviator/Flight Officer) than to just become a ground officer.
Will the selection officer look at grades?
Yes, but I can't remember what the min GPA was for application. I wanna say maybe a 2.5 or 2.0?? It's been a while since I helped out with the OSO.
Also would you all recommend Offiver Candidates Course or Platoon Leaders Course?
Depends on what you're into. I did PLC and it can be nice to have a break from training, but considering that you have another six weeks looming over your head can really get to be an elephant on your back. Besides, once you are six weeks into PLC, it almost seems retarded to have to come back later to do half of it again when you are already in shape now (personal experience). When the liberty calls hit, all you have to do is focus on making it from weekend to weekend and it helps get by a bit easier.
When would I take the ASTB?
As soon as you can also. If you don't make it on the first go around, then it gives you plenty of time to study up instead of pushing it right to the end. See answer 1.
Just know, as I am sure that you have probably seen on different posts since you are somewhat familiar with the process, that being a Marine Officer is first and a pilot second. You might not know exactly the difference right now, but they will drill it into you during OCS and again at TBS. You should be there to support those hard chargers on the ground vs get to fly some sexy aircraft (which the Corps does have a few of). Hang around a few Marines and ask them to elaborate and you will get a better picture.
Lastly, on the AFROTC deal. Once you accept your scholarship (or whatever they call it), you are LOCKED in and must serve your time with them or feel the pain of having to pay it all back. The Marines will let you get all the way to the day of your commissioning and let your respectfully decline (except if you take a certain Tuition Assistance program, but talk to the OSO about this). The last the the Corps wants is a leader who isn't fully committed to their Marines or his mission.