In response to your specific questions, just put "ADD Myth" into your google search engine and you will find the answers to 1, 3, and 4 of your questions. ADHD is considered an inherited, biological brain defect by orthodox psychiatry. As for your last question I've seen people on anti depressants as well as ADD medication, but perhaps not ritalin. They probably figure that while on the anti-depressant the depression is fixed so they can take the ADD medication. Who knows. haha.
Just don't expect a psychiatrist to care about your child's (or any child's) health when it comes to medication.
there are alternatives to medicine. It just sucks that your kid is already diagnosed with ADHD. You might have a hard time refusing the medication for alternative treatments.
I don't even believe in ADHD. Someone once said it is like a doctor seeing someone with a cough and diagnosing them with "coughing disorder." There could be many reasons why someone is hyperactive or not able to pay attention. One of them is boredom. One of them is laziness. One of them is lack of proper nutrition. I see people with horrid diets who wonder why they can't focus.
Everyone wants their kid to do well in school. But good grades are not determiners of success. Do not sacrifice your child's health for performance in school. You can work on self esteem issues surrounding performance in school with your child. Just don't "medicate" them. You don't want them to grow up with that kind of mindset. You know?
I wish I could reply as you requested, but I don't know all of these answers. There is a better forum on medhelp for children's behavior and they specialize in this, and they can give you these answers. I have a bias against these medications except in very extreme cases.