First of all, great work! It's really impressive that you entered this essay contest, and must have taken a lot of time to write your ideas up. I certainly don't think I could have produced something like this when I was in high school. Best of luck in the college admissions process, I think you'll do quite well!
Second of all, it's true that there are a lot of mysteries in modern astrophysics---the strange rotation curves of some galaxies being one such mystery. In many cases, some of the most common explanations (dark matter in that case) have some unsatisfying aspects. Dark matter has never been directly detected, for example. So it's great to be skeptical, and to imagine and investigate alternatives (e.g. modified gravity is one alternative to dark matter).
Moreover, if you can show that an alternative theory produces both qualitatively and quantitatively true predictions (quantitative is important---people will demand the numbers match up!), generally speaking the scientific community will be happy to take up your idea with open arms. The scientists I know don't falsify data. If aspects of relativity (which has been tested pretty thoroughly, e.g. time dilation in particle lifetimes) are rigorously shown to not be true, scientists will accept it if your proof is convincing.
On the other hand, while it's tempting to throw up your arms and call lots of scientists dummies that don't understand that their favorite theory is clearly wrong (I want to do that quite often, myself), the reality is usually more complicated than that. And there's a lot of work out there you have to familiarize yourself with to be sure. Some of your claims (e.g. on pg. 4, you claim the sun may have a high abundance of heavy elements) have been well studied. People use the spectrum (light at various wavelengths) of stars like the sun to determine the abundance of various elements. According to empirical results from such spectroscopic studies, the sun is mostly hydrogen and helium.
Also, keep in mind that plenty of other scientists have had a hard time accepting counterintuitive theories like relativity. That's why people have thought them over so much, and tested so many consequences of them. So far it turns out to be mostly right in its regime of applicability, but no one will be sad if Einstein turns out to be wrong---we just want to know what the truth is about the universe!
Lastly, keep asking questions, working hard, and staying curious. It may be that you're right! But convincing people, and yourself, is a much harder task that requires lots of evidence and careful thinking. And remember, at the end of the day, that your task is to find the truth, not to confirm that your favorite theory is true. I am reminded of a great quote by Feynman:
"The first principle is that you must not fool yourself--and you are the easiest person to fool. So you have to be very careful about that. After you've not fooled yourself, it's easy not to fool other scientists. You just have to be honest in a conventional way after that."
John