I think you would be ok to count them, but it might be beneficial to keep them separate in case someone asks. They might want to see that you've done shadowing separate from your job as an assistant. I'm sure you are learning tons as an assistant, but that is just how your doctor(s) does things. You would benefit greatly from doing shadowing separate from your assisting to see different perspectives and ways of doing things.
I would say so! It is time that you are spending learning more about dentistry and seeing actual dentistry done. I don't see why it wouldn't count. If the school specifically asks how you were able to get so many shadowing hours, you can explain that you were an assistant. I can't see why it would be a problem.