Welcome to compost tea....
My suggestion would to begin with a soil test so you can determine the organic matter % of the soil and at least some of the basics of your pH and nutrients.
From there, you can formulate a basic plan of action. It sounds like what you have going on there may be beyond what compost tea can offer. First, CT will help to build organic matter, but this is a very slow process (your clients may not have the patience to wait for this).
Good soil structure can only be achieved and maintained by addressing fundamentals such as adding organic matter (5%+ is widely accepted). Without this, you'll be constantly seeding, aggressively adding soil amendments and turning up weed seeds every time you aerate (which will be often).
Chances are, your soils contain sufficient Phosphorous; It's very likely that you don't have the biology (beneficial fungi) to mineralize it into a plant available form. Remember that most soil tests only quantify available
nutrients in soils.
No doubt that we can help you with your compost tea operation in many ways. If it's done correctly (having the right knowledge, equipment and substrates), the results can be quite dramatic. CT is not a silver bullet. Think of it as a tool in the tool box. There's a time and place to use every tool.
Others here can do a better job at answering your second series of questions.