The short answer is yes, you can be whatever you want to be.
I know I'm new here, and I'm going to ruffle a lot of feathers with this post, but so be it. I've been thinking about this post and their responses, and all I see is discouragement. That, to me, is far more discouraging than the trials to be faced after graduation.
This kind of discouragement stopped me when I was in college the first time. "You won't make enough money..." or "It isn't practical..." or, or, or.
Butbutbut. Isn't this your life to lead? You may get into college with a major of Music and a minor in business administration and discover two months into it you are an architect. It is better to try it and find out the hard way, than always wonder about it. Yes?
I have known music ed majors who had two years on the cowbell and six months on the piano and decided music was their "thing". They are now very successful and popular band directors. (One of them is blind. Talk about not sight reading!) I have a friend who got a composition and conducting degree from MIT (no lie), who is now a software designer.
It is possible for each and every one of us, no matter our level or time at the piano, to be the best the world has ever seen. We have to work hard, practice our hands off, and get a lot of good promotion, but anything is possible.
Struggle to find work? Most school districts around here are screaming for band directors. Competent ones. Mature ones. Are you willing to relocate for the sake of your job? That opens up a lot more possibilities.
As far as "What am I really up against? What's the best way to go about teaching myself everything I need to know? What, specifically, do I need to know? What should I focus the most on? Any books you recommend? Sites?"
1. You are up against a lot of people who will try, at every turn, to discourage you. They will list a hundred and one reasons. It is your CHOICE to listen to them and heed their advice or not. You are facing many, many hours of practice, lessons, studying...so you are up against time, of which there is often short supply.
2. The best way to teach yourself is to get a good teacher and learn from them. Many a bad habit is formed when one does not have proper instruction; these habits are very difficult to break and relearn.
Music is a language. One we can all *hear* and appreciate, but not everyone can read and interpret the written "word" of it. Get someone who can already read the language and let them teach you the grammar, font, and punctuation. I suggest a private instructor for now, then college. Depending on how serious you want people to take you and how hard to you want to work for their respect, that paper can go a long way to getting your foot in the door.
3. You need to know everything you can fit in your head. Seriously. Know where you want to go after you have the pretty paper that says you know music. Know your options for where you are going. Take a map.

Know what lies upon the path between where you are know and your final destination. How many hours of college? How many hours of practice? How many tears? Failed finals? Hairs you pulled out of your head? You also need to have enough of a grasp of theory to go from a quarter note run to an eighth note run without thinking about it. Instant recognization of treble and bass clef notes (3rd space up on treble is a C, second line down on bass is an F, that kind of thing). The ability to read a piece of music and have a general idea as to what it should sound like even if you can't pound it out on the piano with perfection the first time you try.
Guess what? They can teach you that theory when you get to college if you take basic and general music. You can always bump it to education or performance at a later time. Things like tenor clef, alto clef, transposing, transpositional instruments, conducting, you will need those later, but starting out?? Let someone teach you. The internet can be a good resource, but there is a lot of conflicting information to be found.
4. Focus on your passion. After you survive my instructions for #3, if you think it is still worth it, focus on the passion. Additionally, work on theory on your current instruments and do not worry about picking anything else up at this point.
5. Sites. Well, this is tricky. I could throw out a hundred different sites based on a hundred different areas, but only you know where your weaknesses lay. What can you already do, well? What do you do poorly? What do you wish you knew better?
Start talking to colleges now. Talk to local ones (don't expect to get into Juilliard right away) and sit down with their major instructors. Once you find a school where you are comfortable, find out what their admission requirements are. Is there an audition? What is the audition for? I ask the last one because the college I am going to requires an audition for scholarship purposes only. If you don't want the scholarship (seems silly to not aim for it, at the very least) you do not have to audition. I know some schools which require you to audition for entry. Ask questions. Ask a lot of questions. Keep a notebook and write down the new questions as they come along, check off the ones which get answered.
Don't let time be a factor in picking your school. I waited 17 years after I graduated high school before I got serious about my education and direction.
Lastly...do not tell me the sky is the limit when there are footprints on the moon.
Good luck!