1. Yeah, although it may be better to think of してはいけません as a single unit meaning prohibition. The meaning you got pretty close: "don't be a bother to other people."
2. 一人・で・したら, and yes, it means "how about doing it by yourself?"
3. This one's a bit hard to understand, but if you think of it in terms of someone (or people) expecting someone (or thing) to show up, and when they (it) becomes visible (or their presence otherwise becomes known), the one(s) expecting it shouts 来た!it may be easier to swallow. Now the question for me becomes whether this 来た phrasing means that the car(s) was expected. I'm pretty sure I've only ever heard this kind of 来た when someone (or thing) was expected, anyway.
4. それ is whatever the pros and amateurs do, and で indicates that it's a means for earning a living.