As for summer seminar, you must first realize it's main purpose. To give the individual a taste, albeit a small one, of what the academy is and what they might be getting into. As much as you or anyone else may believe that the academy is the school and career you want out of high school, the truth is, you have absolutely no idea what you're really getting into. No one does. Not until you're actually there. There are those who have applied to the academy and have said: "I've wanted the academy my whole life". Then, when they get there, they realize that it's nothing like they thought it would be. Some quit. Even some who said it's what they've wanted their whole life. So, the summer seminar is an opportunity to get a taste of what the academy will be like. There have been some that come back from the summer seminar and are more pumped up. There are some that say; this is definitely not what I want.
Will it have any positive affect on receiving an appointment? In your application; no, none at all. As you already said, it's more difficult to get accepted to summer seminar than to the academy. All of your information when applying to summer seminar is self reported. Most people don't have true ACT or SAT scores. Plus, there's just not that many slots for it. So a very large percentage of those who do receive appointments to the academy never even went to summer seminar. The academy is not going to hold it against anyone's academy application who didn't get to go to summer seminar. Likewise, they won't give additional points to someone who did go to summer seminar. Again; it's for your benefit.
Now; indirectly, will it benefit your quest for an appointment? It can. How? We are all humans. Therefor, when your ALO interviews you, or your MOC interviews you for a nomination, etc... It will show us that you are dedicated to attending the academy. That you went out of your way to give the extra effort by applying for summer seminar. But honestly, I will give just as much positive praise to the individual who applied to summer seminar and didn't get accepted. The fact that you applied; whether accepted or not; shows you have a strong desire to attend the academy. So indirectly, it may help you get a nomination or possibly receive a higher score during your MOC or ALO interview. But actual affect on your appointment? None.
As for your class schedule, it is always best to take the most challenging classes you can, and to do well in them. A 3.8gpa in the IB program or with a lot of AP classes is better than a 4.0gpa in the "Minimum Required" classes. Combine this with the ability to do sports; have leadership positions; volunteer; be part of clubs; etc... will make you stand out the best. Put it this way; I've seen 4.0gpa applicants who had all the AP/IB classes available, who didn't receive an appointment because that's all they had. They didn't have sports, leadership, volunteering, etc... I've also see those who played 2-3 varsity sports for all 4 years, captain of some of those sports, member of clubs, other leadership positions, volunteering, IB program and/or AP classes, with a 3.7 gpa and 30ACT/2000SAT who did receive an appointment.
But remember; there's also those applying who not only had 2-3 varsity sports, captain of the sports, class officer, Boy's State, Volunteering, other clubs, AND ALSO HAD a 4.0gpa in the IB program or ALL AP classes; and they scored 31+ACT and 2100+SAT. Whether you get an appointment or not is totally dependent on who your competition is. And that is something none of us here know. That's why we can't answer the "What are my chances" type questions. Obviously, there are some in the "No Brainer" club who always had the 4.0gpa in the IB program, class president, varsity letter/all state/captain athlete, boy's state, and all the other attributes; triple qualified; along with the kick butt ACT/SAT scores. Probably close to 10% of the academy is made up of these individuals. Those are easy to say: "Unless the end of the world comes, you're pretty much guaranteed an appointment". But that is rare, and even then, not 100%. So just do your best. Take the most challenging classes you can take. Do the best in them. Stay involved in sports, clubs, volunteering, etc... and have leadership roles in these. You can only do "The Best" you can do. And only YOU know if you're giving it your best.
And for what it's worth, this same advice is exactly the same whether you're also in JrROTC, Civil Air Patrol, Scouts, or any other activity. None of these things by themselves gives you an edge. The academy wants well rounded individuals who are diverse in their experiences. The academy is training future leaders and officers. You will be leading the most diverse group of enlisted individuals in the entire country. This is the kind of officer they are looking for. The more diverse you are in your experiences and the more successful you are in your activities, academics, sports, etc... The more you become the type of applicant/cadet/officer the academy is looking for. Best of luck to you. Mike....