Don't include the HS volunteering in the Experiences section unless you continue the activity into the college years. You may refer to it in the Primary Statement though.
The year of planned US hospital volunteering should be fine; aim for 150 hours total.
You need US physician shadowing. You can list what you did abroad, but it should not stand alone. Aim for 60-80 hours total.
A year of research seems average. With two years you'd have a better chance at schools that value research. Any chance at a publication?
Keep up with the community service so it will be recent when you apply. You're doing fine with that so far.
TA can go under Teaching. It's good to have.
Leadership is good too. Be sure you do more than lead meetings for your club. Get some new initiatives going. BE a leader. Have leadership-y activities to report.
With a cGPA of 3.7, aim for a 31+ MCAT. Higher score gets you consideration by more schools. I think you're in good shape despite the two Cs.