I'm getting into this a bit late, but here goes.
Honey, I've been doing the horse thing for almost 20 years now - if there's anything I've learned, it's that the amount of time involved with horses reflects very little on your own abilities. I know people who have had horses for 40 years or more who still barely know which end the poop comes out.
You need help. I know you don't want to hear it, I know you're very confident, but you are coming on here seeking help and the issues you are describing are simply not things that we can help you with over the internet. It's upsetting to be told you don't know as much as you think you do, sure, but the results are speaking for themselves here. Accept it and move on, you can take the advice given here and work with a capable trainer to gain the experience you need, or don't take it - but don't complain to us if it doesn't work out for you.
The behaviours you describe in this 3 year old (and the 4 year old) are unacceptable. The 3 year old is not good on the ground if you can't handle the hind legs (and I don't mean being able to touch them before he jerks it away, you should be able to pick up the hind foot, do with it whatever you please, and put it down when *you* decide to put it down.). If he's bucking, then he is not respectful under saddle. If your 4 year old is having tantrums then the lack of respect is the issue, not his size. My fear is that your 3 year old is going to go in the same direction, as you appear to be missing the initial signs of disrespect and don't do anything until the situation is severe (ie tantrums, lying down on the trail, bucking, these things don't come out of the blue). You can't learn how to see these things on the internet.
You seem to be very focused on the 3 year old's past. Let it go. If you work with this horse focused on how the amish were 'mean' to him (and were they really? Did you see them work with him? Or is this the excuse you've made for the behaviour?) you are going to hinder the progress he makes, if he makes any at all - the point of desensitizing is for him to not see these things as a big deal, and that will not happen if you give him excuses for being afraid. While knowing the possible traumas in a horse's background is useful in deciding how to train, fixating on the past and making excuses for negative behaviour based on what may have occurred isn't going to get you anywhere.
As for feed, it's hard to say without being there - another reason to get a trainer! I don't like oats and other grains, and if it were me I would remove them from his diet. Alfalfa may also be making him energetic, most horses are fine with it but some get wired, but since he is growing he needs the extra nutrition, and since he's young and green the energy is simply part of the territory. Free choice grass hay supplemented with alfalfa if need be, and/or a feed designed for growing horses that is not high in NSC may benefit this horse. I am not recommending this, as I have not seen the horse personally. Get someone who knows what they're doing to assist you, and do your own research on top of that - OUTSIDE of the forums - we're talking books and scientific articles.
Best of luck and I really hope you find professional help. You may balk now but it can only help you in the long run.