I'm so sorry. I think you will find that just by getting this all out, vented into cyber world you will start to feel better.
We deal with major food allergies here too. My son can not do wheat, dairy, eggs, soy, lentils/beans, seeds, nuts, chicpea, pea and dog. I think that is it? I might be missing a few.
My suggestions for the allergies is this. Take a few moments, when you can and read everything you can at enzymestuff.com. Especially considering that your children have intolerance's and not IgE related allergies you will probably see a drastic improvement in tolerated foods. Also, if you haven't all ready get them all on a high quality probiotic.
Even with my sons allergies when he was nursing he stopped being able to nurse. He'd get hives and have a major reaction anytime he nursed off me. Even if I wasn't eating any of his known foods. I had to put him on an amino acid based formula for a few months because he couldn't seem to tolerate any food proteins of any kind. This kid was reacting to things like applesauce, carrots, cantelope, everything! I took enzymes for 2 weeks (at this time he'd been on the formula for 6 weeks) and decided to try him on nursing. He didn't react at all. I got him off the formula (which he wasn't tolerating that great either) and exclusively back on breastfeeding. This was when he was 11 months old. I was able to start eating all of the foods he was allergic to and he no longer reacted at all. No more hives, anaphylactic episodes, no digestive issues, poo issues, skin issues, nothing. I continued nursing him until he was just over 2 eating any type of food I wanted.
My plan now that he is a little over 2 is to start working directly on his own system with the enzymes. People see amazing differences with them and are able to begin eating foods they could previously not tolerate. The problem with allergies from my experience is that they feed off of each other. Allergies cause inflammation in the body and anytime you have inflammation your body is more reactive in general. My son will start reacting to foods he normally can eat just fine if he's been ill with a fever. If you can get the inflammation under control people tend to become less reactive. My guess is that if you get them on enzymes for a few months and slowly try one new food at a time they will be able to add back in many foods that they could not tolerate previously.
BTW, I didn't have to stay on the enzymes. After only 3 weeks of use we never needed them again.
In regards to some of the other things you mentioned. Big hugs. My 2 year old was/is so similar. He is mellowing out a lot but my goodness there were days when I wanted to send him back. He has been soooooo challenging to parent with the allergies and combined personality traits plus just being a destructive boy.
Trust that it will pass. Pray for him. Pray for you. Try to love him. Here is what has sometimes helped me. I have an ergo carrier, they are a backpack style carrier that you can use on the front or back. I wore him for awhile when I could to keep him out of trouble and give him more mommy time which helped. This isn't always practical and it won't magically change him but if you are not getting lovey time with him try to work it in. Ironically this little trouble maker has a very big need for physical contact with me. Bath's are also helpful (they keep him away from others). If you have anywhere in the house where it is manageable make a no-fly zone for your toddler. I'm guessing your son can climb over baby gates like mine but allow a room in the house where he is not allowed (until he is out of destruction mode) and let the other kids stash their toys or other objects that he'd destroy in there. Add a deadbolt to the door or a handle that locks and let them play alone in peace. It isn't ideal and I wouldn't normally suggest separating family like that but your older kids deserve to have their belongings respected and your 2 year old needs a reality check that his behavior will determine what privileges he will receive.
I found that within 4 months or so my previous nightmare of a toddler is now much much much better.