You have given yourself some big challenges so I can see that you are learning fast.
Here are some general principals
If you can get around your area you might spot fruit trees that are doing well or poorly. If you see a lot of crappy plants of a variety--best to cross it off your list and maybe go back to it in the future.
Watch the Gardening Australia show. Josh Byrne in WA is great with his clear explanations.
Research;
A crop before buying--find out the best season to plant and what to do to the soil beforehand.
On the net find out about sandy, alkaline soils and hard water--with a lot of calcium.
Boron toxicity as its similar to salt damage.
Major and minor nutrients and how they become available or unavailable according to the soil pH---there are lots of charts on the net.
Learn about salt tolerant crops eg olives and pomegranates are pretty good.
That might save you some money.
It sounds like the stone fruits are struggling along with the fruit fly. Consider culling them to save energy and resources. Accept the loss as a learning experience. There are other aspects to consider;
Hot drying winds damage sub-tropical and tropical plants.
Learn how to apply acidifying fertilizers--eg Sulphate of ammonia and Urea are good in cool weather provided a small amount is used.
Look at how you can protect the garden with hard barriers or hedges. Check out what you see around your area.
Next time I'll look at your mango as its worth saving.
