8/8 people got into Peds and I think 3 into ortho this graduating year. If you really want to do Peds stony brook has you covered. Ortho is tough just about anywhere and I can't really comment on ortho, since I have no interest in ortho.
Okay as someone who actually attends Stony Brook, heres my take. It'll definitely be cheaper here, and I'll try and answer your questions regarding stony brook only. I did live in the city but I don't attend Columbia so I have no input on that school.
1. A's are a little challenging to get but completely doable with a normal life. You just have to put in the effort to get those. Some classes are easy As, some classes you will barely scrape by with one. So its a healthy mix like in undergrad.
2. Theres plenty of research to do at Stony, however you have to find the professors and reach out to them. You wont be handed anything, and since you are so proactive in ECs that shouldnt be a problem for you.
3. To do well, you have to not compare yourself to others and when things start getting bad you just have to pull through. I remember getting 0s on quizzes and I just laugh it off. Why? Because 1 grade isnt going to define me. Will there be people in the class that just look untouchably smart? Sure, but that's what happens when you put 44 people with high Dats and GPAs together. The ones that start to struggle are the ones that allow 1 grade to nosedive everything else. If you've met adversity in the past, dental school is going to be just fine.