I skimmed this whole thread and didn't notice this advice re: high magnesium. You need to add calcium!!!
Calcium will also serve to "open up" the soil. Especially with clay, that will make a big difference. I farm in the PNW where my soils are mostly clay. The high magnesium is mostly a problem in so much as you want to have calcium and magnesium in balance in your soil. Adding calcium will help both with the mag and the clay soil
I recommend bone meal because it will break down quicker than any mineral form of calcium. Don't be shy; it's hard to add too much in your setting. A few other thoughts:
1. Def got to lime. Get that ph right where it needs to be. If you don't fix ph, your orchard will never thrive
2. Don't add softwood mulches--they'll acidify the soil and don't promote fungal growth. Use hardwood mulches, as they promote fungal growth, which you want. Straw and hay work great too. Maybe you can find some old/moldy hay for sale or free?
3. Optimally, your soil would've been balanced and as healthy as you could get it before you planted the trees. It may hurt your pocket book, but you might consider replanting most of it. Being stunted early in life has knock-on effects on orchard plants that can't be fully "fixed" by better soil and care later on. I've made this same mistake and am slowing re-planting some trees as am able to afford to. The new ones are doing much better.
