If your questions is if you can make the seat reservation you want in any order, the answer is yes. There is no specific order in which reservations have to be done.

I have a 15 day over 2 months pass so if I make a reservation for mid September I can still make reservations for earlier in the month without a problem?

Yes-as anna has already explained. REServ are completely apart from the trips-these you only put in app later-preferably as late as possible.

Note that in several cases you are again the early bird-so early that in some systems its not even yet possible.

Thanks