Forget about bodyfat percentage - forget measuring it, forget tracking it, forget caring about it. It's useless for athletes and serves no purpose in the real world.
I'd say yes and no. It's always best to prioritize one thing at a time, especially when the different goals are achieved by almost opposite methods (speed training is very different from training for size, for example). But on the flip side, you have the advantage of being young with plenty of time.
You don't need to be doing the plyometrics/agility work three days a week for all 8 months. Again, prioritize.
I'm all for laying out a general plan, this looks like you're setting things in concrete a little too much. There should be some wiggle room down the line. Have a general idea of where you want to end up, but try planning 2 or 3 months at a time and adjust as you go.
Also, I don't really agree with this kind of setup since "mass, strength, power" isn't one goal to shoot for, so it doesn't really help to narrow down the type of training you'll be doing.
Here's how I'd kind of lay out the upcoming calendar:
June- June's over, so it doesn't matter.
July- Bodyweight conditioning
Aug- Focus: Size
Sept- Focus: Size
Oct- Focus: Strength
Nov- Focus: Strength, power
Dec- Focus: Power, speed/agility
Jan- Pre-season. Focus: Speed/agility, conditioning
Feb- Ball season starts
Some categories may overlap, especially since you'll see improvements in several things at once because you're a beginner, but having one thing as the basic priority helps to organize your training.
I don't know what your training looks like right now, but I'd start with a few weeks of higher rep bodyweight stuff before transitioning into a basic routine for 6-12 weeks. (Dan John's "Southwood program" is a good one.)
No, that would be an very inappropriate program for you right now.
Forget about fast twitch or slow twitch muscle fibers. It's a level of detail you don't need crowding your brain just yet.
In addition to Joe DeFranco, guys like Eric Cressey and Tony Gentilcore (both work with a ton of baseball players), Dan John, Mike Boyle, and Jim Wendler all have a bunch of experience working with high school athletes. Anything you can read by them will be golden.
How'd yesterday's game go?