I don't really understand what these "independent companies" are. We went to Gillette in St. Paul, and through the developmental neurologist we saw there, we got a 6 hour psychological eval, an audiology eval, a vision eval, and an OT eval. We had to pay our copays, meet our deductible, and take care of the coinsurance (just like any other major medical expense, like giving birth or having surgery.) Our regular pediatrician, who sent us to Gillette, also gave us the option of going to Fraser or Children's - we chose Gillette. It took a full year to get a diagnosis, and, yes, there were tons of forms and a lot of waiting for appointments. But in the end, we were totally confident in the diagnosis (ADHD), and had developed a detailed plan of action (which included both meds and OT.) My understanding from our developmental neurologist is that meds alone are not nearly as effective as meds plus OT. Your son needs to learn to pay attention to his body, to know when it needs something (more stimulation, less stimulation, movement, stillness, etc) and to learn tools to get those things in appropriate ways. The meds don't change these physical needs (*nothing* will, it's brain chemistry, it's just the way your son's body works), but they can help his body learn to be patient, and to wait for the stimulus that it needs. OT can help identify what kinds of stimulation his body is looking for, and give him a whole set of tools to find that stimulation in appropriate ways.
There are so many other things that can be going on in addition to OR besides ADHD. Getting the right diagnosis is crucial, and worth the time and effort. You either spend the time now and get the right diagnosis, or you waste time on interventions that aren't ever going to help, because they don't address the root issues.
If you have more questions on what insurance will cover, call your insurance company. They are more than happy to point you to certain providers that they will cover. It is in the insurance company's best interest for your son to get the right diagnosis, too.
P.S. Once we got our diagnosis and found the right medication dosage, we went back to our regular pediatrician, who now writes our prescriptions and signs off on OT for insurance.