I agree with SlowPop’s comments. Also, I don’t think it’s impossible to get a good sGPA in CC as long as you take on a major that you are woefully ill suited to, and even so, if you push yourself (I know someone doing bio and applied math), it can turn out well. Due to health reasons or whatever, I definitely didn’t go as hard as some of my classmates but I got above what you’re worried about.
Mowsh bio is infamous for its difficulty; personally, I think if you can think analytically, you’ll be fine. Mowsh bio vaguely tends to be curved to a B, Orgo to a B or B+, premed physics I wouldn’t know, Calculus to an A- or B+, Stats is similar to Calc. Is it a cakewalk? No, you definitely need to put the time in, but I think that many of the people who were premed when I started are still premed now, and the ones who left it, typically went into more lucrative careers in finance or consulting, and Columbia will give you far more options than PSU.
Overall, grading is fine. With core classes it varies, but usually you can get a read on the professor and how they mark and try and switch sections if you think the situation is dire.
Stress definitely exists but I think a lot of the problems college students have with it is due to mismanagement of time and not taking good care of themselves. It’s more about maturity than not. If you are focused, start volunteering early (and there are many opportunities in NY), and get your MCAT done by junior summer, you don’t need to take any years off. If you take it senior year, you should only have to take 1.