That's a good question. My wife came up with an idea for an app while she was doing her fieldwork as an occupational therapy graduate student. She worked with a student who struggled with handwriting. We ended up building an MVP that would allow the student to complete his school worksheets by typing on an iPad instead of writing with pencil and paper. She showed it to the student's occupational therapist and teacher and they loved the idea.
We put it in the app store and reached out to a few therapists and teachers. Word spread that it was a life changer for some students. With more than 1M downloads, I'd say we struck a chord. We reached out to those people who have a vested interest in seeing the child succeed or minimizing the pain and frustration for those same people. Our app was designed for a 5th grade student, but it's being used by students of all ages (Kindergarten through college and beyond).
I'm not sue we'd be ready to pay for a service to help with user testing because our revenue isn't large enough, but I could certainly see value in it. To answer your questions specifically:
1) We usually communicate with the students' therapists, teachers and/or parents. Or more likely, they communicate with us - looking for a solution to help their student. I usually start with a generic open-ended question "How's SnapType working out for you?" And they share what they like and what they wish it did. It's incredibly valuable information.
2) The people we communicate with are not always technically minded. Nor are they user experience experts. We have to take what they say and read between the lines to understand what they really mean. Some people respond promptly, which is great. Others take their sweet time (or never reply), and that is frustrating.
3) From most important to least important: A, D, B, C
A few final notes:
As an app maker, I would love to see video of my app being used by students. Where they tap, where they get confused, what actions seem illogical, etc.
All the best!