I don't think the instructor did anything wrong in the second scenario. It was being billed as an intermediate class with pre-knowledge required. If you take a course without meeting the pre-requirements for it, then you're not going to be able to keep up with the class and it will affect the people around you. If nothing else, the instructor has to spend more time teaching you the basics, when the whole point of more advanced classes is to allow the class to do more without having to cover the basics. Since you said the instructor said it politely, I say she was in the right in that situation. In the future, if you aren't sure if you meet the prerequisites for a class, ask.
The Zumba instructor sounds really mean, though. I wouldn't want to go back to that class either!
The yoga instructor didn't explain anything to me during class, she just came and talked to me after class so she wasn't wasting everybody else's time on me. She also didn't see if there was anybody new in class and offer a brief intro as many instructors do. I don't think the description of the class was very accurate, the requirement was 'having taken 10 yoga classes' and not actually billed as 'intermediate', it should have also stated that participants 'be familiar with yoga poses'. I had a little experience with yoga (about 10 classes over my lifetime at that point) and didn't realize that she would call out poses without giving an explanation of them. Maybe I should have erred on the side of caution and not taken the class since I barely qualified, but the description of the class wasn't that accurate to me. I wouldn't deliberately take a class I wasn't qualified to take because I personally don't enjoy being lost in class and probably should have asked, but hindsight is 20/20. While I was in class, I decided to stick it out and follow the instructor as best as possible rather than gathering my stuff and leaving in the middle of class (which may also be rude and disruptive in the small class). I don't think the instructors manner was impolite, but hearing I was messing others up and not to come back was hurtful and seemed like she was making it about me instead of saying the class wasn't a good fit and the other participants had been attending for a while and knew the poses. I may be oversensitive, but it is never fun to get the vibe of 'you suck, go away', even if it is said politely.
The Zumba instructor did not make an introduction to new people. As I stated in the OP, I didn't see the woman she called out. If the woman was doing her own thing (as opposed to simply not being able to keep up), I would think that was rather rude to do in a group setting. Either way, I think the instructor should have called her out if it was a safety issue and said something helpful like telling her to march in place and be careful of those around her instead of just scolding her and telling her she was messing people up. I was really surprised that woman stayed in class and didn't just get her things and march out.
Most instructors in classes I take ask if there are new people and give them a brief introduction and tell them to march in place/go in child's pose if they can't keep up with the class.
I am also queen of the uncoordinated and un-graceful, and have also taken a variety of classes over the years, regular aerobic, step, kickboxing, etc. My issue, esp in classes where you need to something separate with your arms and legs at the same time, is that I get confused. My brain won't let me do both at once, so, say for example, in step class, I would simply do the legs, while letting my arms hang. If its a class I'm not familiar with, I will hide in the back so as not to get in anyone's way, if i can help it.
I am the same way. Anything highly choreographed is hard for me to get the hang of. I just look at those step classes with them stepping on the bench forward, turning around, and moving their arms at the same time and my brain gets confused. The few times I have tried it, I find myself trying to think myself through the moves and of course falling way behind the class. With kickboxing drills, if I practice a few times I will tend to catch on, but step is way too choreographed for me and doesn't click with me even after several times of doing the move.