* Carbo-loading the night before isn't too important. Eat a good-sized meal, and plan something for breakfast as well. I like bagel & banana or something like that. I definitely second the earlier suggestion of getting used to eating real food on rides. You're going to get tired of whatever pseudo-food you're eating/drinking, and having some confidence in eating real food will open a wide variety of options to you.
* Riding your best 2-3 hours after a meal sounds to me like you're eating larger meals, and that you may be experiencing peaks/valleys of energy. You'd really rather have a constant supply of calories, without stressing your digestion too much. I like eating small, and eating often. I set an alarm every half-hour or so to remind myself to consider eating a few bites. I don't force myself if it's not right, but on average the reminder helps me to do the smaller/more-frequent thing. I wouldn't say I really have "meals", just the constant low-level intake of calories.
* I wouldn't recommend clipless for you. You sound somewhat new to cycling (pardon me if I'm wrong), and I think that when you're on a crowded ride like STP you are going to want as much bike handling confidence as you can manage. Clipless pedals tend to freak people out a bit and be less stable/predictable when moving at low speeds (stop lights, speed bumps, rest-stations, etc.). You'll do fine with toe clips, as long as you're comfortable. If you're not comfortable, then I'd sooner recommend you look at different shoes or cages, rather than a clipless system.
* Electrolyte capsules are good, but you need to spend the time learning what your body needs lest you over/under use them. Temperature/weather greatly affects the rate of electrolytes you may need. I often open/close my hands several times to see if my fingers feel like they beginning to swell (I think of it as them feeling like sausages), in which case I slightly increase the rate of intake.
* Bike handling skills in groups are really important. If you're not comfortable riding in groups, you're not going to enjoy your trip. The ride-related cascade training series is one option to find a group. Learn how to be safe when riding in groups, how to keep up with a group, etc. (I'm not saying you have to do pace-lines, but you should be familiar with the ebb-and-flow of a group moving through intersections and traffic and that sort of thing.)
* Yes, back-to-back training is important. Sure, it's for your endurance or muscles or whatever. But really it's for your brain. You're going to have been on your longest ride ever @ 125 miles, and then you're going to have to get up in the morning, get all your sore bits mounted back on that saddle, and do another long ride after that. You need to be mentally prepared for it, in order to keep your enthusiasm up and enjoy the 2nd day too. So do whatever your long training ride would be on Saturday, and then on Sunday get out for at least 10 miles or so. Enough to loosen up your muscles and get back in the groove; and practice being happy about it and looking forward to it.
* Yes, your bike sounds too big. You should be able to stand over it without having to lean it. Either get that fixed soon, or commit to riding the big bike. You probably don't want to switch bikes right before the ride. One thing to consider with a larger bike, is whether it's making your reach too far forward to the handlebars? Having to reach too far could put more strain on your back, and possibly also your neck if you have to hold your head up at a sharper angle. These sorts of things can go unnoticed on shorter rides, but can cause real trouble on longer rides.
* I personally don't like spending too much time in rest-stops. 5-10 minutes tops. Grab some food, get some water, get back out there and eat on the go. I find that long stops just make it harder to get started again on sore muscles. So I'd recommend you consider minimizing rest-stop time.