Pre-reqs

I honestly can't imagine that being the issue that makes or breaks you. I can imagine that there would be some benefits to it--like additional breadth to your experiences as an undergrad. Honestly, I think you would be just as likely to come across a prick that cared as you would be to come across a person who attended different schools in the summer themselves, and found it rewarding. If you want to do it, I say do it.

On the topic of prereqs...

I haven't taken calculus II yet, but plan to take it next semester (had AP credit). The same with a second English course (no AP). Will schools that require 2 semesters of each allow me in, provided that I assure them I will have their pre-reqs done by matriculation time?

Yeah, there's a section on the AMCAS for future courses. Whatever you put in there will qualify you for a school's prerequisites. If you forgot to put it in there, they will reject you right away (in theory at least; I bet most schools are pretty lazy with looking over the details).

If you don't fulfill them and you get accepted, it is grounds for an immediate rejection. And they do see your final transcript once you graduate.

On the current/future courses I entered this semester's courses, but not next. I guess what I'll do is call up each school once I see AMCAS is transmitted, and ask for advice on this situation.