On a 50 mile ride I don't think what you eat or don't eat/drink the day before is going to bother you, unless your body fat is only about 7 percent. Maybe not even then.

Caffeine? yes/no/maybe? For me caffeine is only a problem because in the form of coffee, it makes me want to pee. Doesn't seem to be a problem in the summer because I guess my body realizes I need the fluid for sweat instead of urine. But in the winter when I don't sweat it's a significant issue trying to undo all the layers of clothing to let loose on the side of the trail.

If the other riders are fast, you are going to use your Glycogen reserves. You body can only replace Glycogen at a certain rate, so I think it's best to start out with carbs in your drink as well as eat carbs during the hours before the start.

But don't use stuff you have not used on short rides to see if your stomach can handle it while riding at a fast pace and at max effort.