Thanks for the continuing responses (are you guys up waiting for kids to go to sleep too so "Santa" can come?) As I said in a later response, we did feel guilty enough and decide to bring DS12. Should we have? I really don't know. I'm glad DS11 is excited (yes, we finally told him) and DS9 seems to have accepted the idea, but DS12 is REALLY getting on my nerves. After yesterday, when we were actually playing the Magic Kingdom game and he kept teasing and pouting and teasing, etc., etc., until I finally sent him to his room and as he went to it screaming at me threw a jack (you know, for playing jacks - I have no idea where it came from because I don't think my kids have ever played jacks, but I digress..) which bounced off the game board and hit DS11 right in the face below his eye. He started screaming and I went in and smacked DS12 in the face. Yes, now you all know I'm a terrible parent. I don't know the last time I hit him (and I haven't very often - usually only when like this I'm already at the end of my rope with him and then he hurts someone). He continued screaming "You have NO RIGHT to hit me!" and I apologized and closed his door where he sat and kicked the door for awhile as we continued playing.
Anyways, we later had a long talk and I conceded I was wrong to hit him but he was wrong to throw something in his brother's face, not to mention what led up to that. I told him if we had another day like this one that he would pay me back the $167 airfare (he has more than that saved up) and I would cut my losses and still leave him home with Grandma and he wouldn't go to Utah either. We were all very upset. I've been really sick the last couple days with still stomach cramps, sore throat, and head cold and this (& lack of sleep) probably led to my frustration/melting point too. But today's been a much better day and I think he realizes that I'm not willing to put up with this. We had a long talk later with him and DS11 (the problem is with those 2 together) and pointed out that DH & I loved both of them and when one of them teased, hurt, etc. the other it hurt us too, etc.
I told them I'll get a possession arrow (like in basketball) and every time they fight, rather than figure out who's right (yeah, that'll happen), whoever's next on the arrow, promptly leaves (the ride, line, pool, table, wherever we are). They laughed, but I hope they get the point.
To address some points brought up. As far as school, although they are beginning a new semester, they don't have a lot of new teachers. DS12 only switches one class - from gym to Spanish. He had the same teacher for Spanish last year and she loved him (and vice versa) so I really doubt there'll be a problem there. I already talked to or emailed all the rest of his teachers and they all told me what a great kid/student he was and go and have a great time and that there'll be no problem making up the work. So he doesn't have any new teachers. DS11 only changes one class as well - from Art to Health. He doesn't know this teacher and she's actually out on maternity leave now so I can't contact her yet but will before we go. But I don't think health (a 10 week course) is super critical. I talked to his other teachers and 4 said similar things as DS12's (he's a great kid, what a great opportunity, he'll be fine) and 1 said those things but also "However, he'll miss classroom discussion which is irreplaceable". That was in science, but he has an A in there so I think she'll work with him.
It's hard to say what DS12 would want to do at WDW. My kids are all thrill seeker types so even DS9 is most looking forward to RNRCoaster (which he was too short for 4 years ago) and I don't think there's anything DS12 would want to do that the other 2 wouldn't/couldn't. The water sprites are a great idea - maybe bargaining tool "if you're good enough, I'll rent one for you on the last morning". Really though, I won't be dragging him on say Dumbo (my kids were all thrilled that it would be closed while they were there) and we decided to forego the character meals due to the cost (paying for the older 2 as adults after Jan 2). Right now we plan on DQ the last morning, but I could tell him he'd sit with me for, say 15 minutes, each time he gets in trouble.
As far as strikes though, with my kids when we try that, they seem to get 3 strikes in the first 15 minutes no matter what. Then figure "oh well, I've lost it anyways". When we go to amusement parks near us for the weekend, each kid gets $20 to spend (on credit). Each time they get in trouble they lose $1. Can't tell you how many times they've lost the whole thing the first day. Or they get 2 strikes and I'm constantly saying "this is your last chance, I mean it" but then either I keep saying that (when they should have strikes) or give them a strike 5 minutes later. I'm serious, especially DS12, just can't behave very long. And this behavior is 100% related to his brothers - teasing, fighting, hurting, taking stuff, etc. If I had any one of them alone, they'd be perfect. Just so you don't think I'm giving them strikes for bad posture or something!

It's usually for doing something that gets one of their brothers upset enough that they're crying.
I got them the Birnbaum's "for Kids" book and have asked them all to read it and list 10 rides they'd like to do (or be most willing to do) in each park, then put a star by the 3 they REALLY want to do. We'll really only have a half day in each park, so hopefully it will help me decide which rides are the most critical to them.
I discussed earlier possibly letting them go off in a pair (I think DS12 is very responsible and wouldn't fight with DS11 if DH or I weren't there to arbitrate). However when I asked both brothers if they'd like to do that (would you like to go explore by yourselves for awhile - just you and DS12?) both adamently said no. They didn't want to go with DS12 because they'd end up doing only what DS12 wanted to do. So I guess that ideas out.
We'd also discussed splitting up - maybe DH & one kid at Epcot one evening, me and the others at MK then switching the next night, but because of hours, MLK day, etc. I don't think that will work. And DH said "are we really that bad of parents that we can't even go to Disneyworld and enjoy it as a family?" If it gets bad, we'll split up in the same park and meet up later!
I don't know that I'd feel comfortable leaving him in the hotel room, because, as pointed out, he'd do something to rebel if he was mad enough. But I could at least threaten him with this since DH will be in meetings all day, much of the time at the hotel, and could theoretically check on him. Bet if I actually did it one morning it would improve future behavior though!
I may give each boy a disposable camera, and as you suggested, tell them their job is to get pictures of other family members being happy! Sorry, but I'm not giving them the digital camera, and if I give one to only DS12, everyone else would think it wasn't fair. Maybe assign a different person each park or something!
So...the point it, unless I sacrifice the airfare, I guess we're stuck with DS12 at this point. I really welcome any more suggestion, especially as far as dealing with fighting in lines, restaurants, etc. They'll fight over anything - if one wants to sit with Mom, so does the other. If one wants to sit with DS9, so does the other. If one wants an aisle seat, the other does too, same for the window or the front or the back or...well, you get the idea. Again, it's the older 2. DS9 is rarely involved and usually gets upset only when someone does something that directly, physically involves him. Otherwise, he's an oblivious, carefree kid! Why couldn't I get 3 of that type?!
Thanks for any help!!
Merry Christmas!