You can't really declare a major online. To declare a new major, go to the advising website https://asitauth.colorado.edu/?pg=aHR0cHM6Ly9hYWNwb3J0YWwuY29sb3JhZG8uZW... and make an appointment to add/change major. You can scroll through a list of majors and choose the one you want. Then, it should give you a schedule of times of the major meetings for that major. You have to go to a major meeting to declare the major.
Yes, maybe not all lab courses, but chem lab courses should be available during the summer. The down side is that they're at 7:30 am. The summer course listings should be available on the CUInfo portal by March, if not already.
There's also Boulder Creek Apartments, I believe at Arapahoe and Folsom. There's also housing along 30th street close to Baseline.
If you're referring to Bear Creek Apartments, yes sophomores and above can live there. If you're referring to family housing, then I believe people who live together must be family members.
Intro to complex variables.
I don't know what the course is like during the summer, but it probably won't be easy to learn that material condensed into only one month, so I'd recommend taking it during the semester.
Focus on anything in bold or italics or a different font color. Also, read captions to pictures and figures because they often explain important concepts. Maybe, skim otherwise, but be on the lookout for things to highlight at the same time. This seems hard, but I think it's doable. Then, you can go back through what you've highlighted when studying. Also, it helps to answer practice problems and question in the book, at least mentally, not necessarily as fully as if you're turning them in for class.
No, I wouldn't count on it. Take the MCDB graduate program at CU, for example. A few hundred incoming freshmen are accepted to the undergrad program a year, but only about ten get accepted to the graduate program a year.
Yes, a little. It's okay, though, because people don't judge people for being out of shape, still.
Take frequent breaks from studying. Drink a lot of fluids while studying. Get a lot of rest, as in try not to study too much.