If your main interest is in philosophy of mind, then a really good grounding in cognitive science is very important. Interdisciplinary research is where it is at now.
Yes. And 100%.
OK, perhaps those answers are too short and uncreative.
But yes, I think reading philosophy, thinking about philosophical questions, and trying to come up with and write about philosophical issues can stimulate creative thinking and improve one's creativity (perhaps not artistic creativity but new ways of thinking about things, new ideas, etc.). I think philosophy students tend to become better at imagining different options and solutions and at writing new types of arguments. I'm allowing my current senior major students to do a creative project to engage with the topic we're discussing (death and the meaning of life), and we'll see what they come up with, but music, drama/dialogues, short films, and video games are possibilities.
And it's 100% because original thoughts happen all the time. Each one of us experiences things in ways no one else has. If you meant original answers to philosophical questions, well, then it's probably much lower, but those are not so easy to come up with.
Good for you! .... There's also this organization you might want to check out, interested in promoting philosophy in high school: http://plato-philosophy.org/lesson-plans-2/pre-college-course-material/
What you clearly need is a slogan and a t-shirt. How about "Philosophy: It's What You Think"?
Well, I think your plans sound great. But of course I would, since I helped develop the Neurophilosophy Track in the MA program in philosophy at Georgia State University (www.gsu.edu/philosophy). I'm not just advertising! (though you might consider our program.) I'm suggesting that your view of philosophy of mind as continuous with the cognitive sciences is a prominent one (and the right one to boot!). Many PhD programs in philosophy (including MIT, but also Washington University's PNP program, UC San Diego, CUNY, Pitt HPS, Indiana, and others) have people and programs focusing on empirically-informed philosophy of mind. Most of them would appreciate your taking some time to study neuroscience or other cognitive sciences. Most of them would allow you to pursue such courses while doing your PhD in philosophy (and some have certificates in cog sci). So, go get an MSc in neuroscience and/or apply for MA or PhD programs that would allow you to get some rigorous training in the relevant sciences. And then join the wave of researchers in philosophy and some of the relevant sciences who see our fields as a joint project aimed at figuring out how the most complex thing in the universe (the brain) does all the remarkable stuff our minds do.
Just a few further thoughts. Many philosophers of physics don't have the equivalent of a PhD in physics, though they do, of course, know a good deal about physics. And while these philosophers usually aren't doing experimental work in physics, what they do is sometimes published in physics journals and often in journals where physicists as well as philosophers publish.
If you have your heart set on being a research scientist, employed by a science department or a scientific institution, then you'll almost certainly need a PhD in the relevant science. But if you want to do research that combines theoretical issues in science with your interest in philosophy, then it's quite possible to do that without a PhD in a science. In any case, I agree with my co-panelist's suggestion: study more science in your senior year if you have room for it in your schedule.
It might not disqualify you at some programs, but it will certainly count against you at most. The writing sample is the primary way of distinguishing applicants' philosophical talents, at least once they have been narrowed down using other criteria (such as coursework in philosophy and grades, letters--though for the competitive candidates, they tend to be equally gushing--and perhaps GRE). A 5-page sample is unlikely to provide evidence that you can develop an argument responding to a particular position that you have adequately and charitably explained. (Of course, Gettier's famous paper is quite short!)
I say all this with empathy--I was a philosophy minor (not major) and did not have a good, long piece of writing to submit when I applied to grad school. I had to use a mediocre, long piece, and was lucky to be accepted in the few places I was. But that was (too) many years ago when the competition was a little less fierce. I would try to work with one of your professors to develop one of your short papers into something more substantial (12-18 pages).
(On the other hand, people should NOT submit pieces longer than 20 pages.)
Peter Smith's advice is dead on. The only thing I would add is that, while you are looking through the Stanford Encyclopedia and Phil Index and PhilPapers (which is a great resource), you look for recent articles whose titles or abstracts suggest that they provide an overview of the debate (e.g., "Recent Work on X"), and then you use the references in those articles to guide you towards other sources. Reading such articles often provides information about which sources will be most useful to you, given your interest in the debate. And don't forget to read the classic works (e.g., most cited) in the history of the debate as well.
Finally, you will make your future self much happier if you keep your sources well-organized (in electronic or real-world files) and if you jot down a few sentences about each article--its main point and how it might be relevant (or not) to your project. My current self is unhappy with my past selves for not being diligent enough about such record-keeping!
let me supplement Eddy's fine response by noting that you will probably have to be very pro-active in making this happen! not only will you get distracted (reasonably) by life, but so will most of the people you're hanging out with, who may not have any initial interest in philosophy anyway! so you'll have to take charge -- for example, start a book club or discussion group at a local coffee shop ... check out 'socrates cafe' on that score ... find organizations that have public events of philosophical import so you can meet more like-minded folks (if you're in NYC you might look up 'socrates in the city') -- make sure your local NPR station carries the program Philosophy Talk (look it up!) and then be sure to listen to it ... organize a lecture yourself -- for example, i recently gave a talk at a bar in New York City that has a tuesday evening literary series ... find such a thing, or start one yourself! .... so don't count on others keeping your philosophy bug alive, you'll probably have to do it on your own initiative ...
You sound like you have a clear picture of the costs and benefits of getting a PhD in philosophy. You should continue to talk about it with your mentors in the field. You also sound like you might benefit from getting an MA (full disclosure: I teach at Georgia State in Atlanta, a terminal MA program). It would give you more background in philosophy and give you a better sense about whether you are interested in going on to get the PhD and whether you have the right skills, background, and demeanor to devote your life to professional philosophy. And if you do have those interests and abilities, an MA will enhance them and situate you to get into a better PhD program (the market is rough, so if you want a job where you have the time and encouragement to do research, you will be much better off going to a highly regarded PhD program). It will be difficult to be an active part of the field (publishing and presenting your work) without a PhD and an institutional affiliation. It can be done but you also risk being perceived as a "crank" even if you aren't one, and more importantly, you probably won't be able to devote the time and effort to the issues to be on top of the literature.
If you do decide to pursue an MA or PhD, I encourage you to do a lot of research about relevant programs, looking at their websites, their faculty member's research expertise, their placement record, and the perception of their quality (which is quantified on the Philosophical Gourmet Report, to be used as one source of information among many).