I've a couple of thoughts for you, based on my experience of shallow (1- 3ft) soil over sloping glaciated rock. I also have poor, washed by rain, acidic soil, cool summers and lots of wind and rain to contend with so all in all not ideal for top fruit. This didn't stop me trying to grow apples, plums and cherry trees and they are still very small several years on. I'm hoping things will improve as the sheltering trees get bigger.....
First I tried to get the most vigorous rootstock I could, figuring the trees are likely to be stunted by the environment anyhow.
Second I planted the trees into mounds and backfilled around them to give as much soil for the
roots as possible. I was most concerned about waterlogging myself, but this did give the advantage of deeper soils for them too.
I'm also grafting onto seedling and crab
apple rootstocks to give vigour.
In retrospect I think I could have buried the trees deeper and hoped for the trunks to
root above the graft point to end up with 'own root' trees.
There's no point wishing for the perfect site, sometimes you just have to work with what you've got.
