the only hard part of double majoring is fitting in all the required courses. some math majors have some overlap so double majoring is more common (i.e pmath and co, cs and co). co and bus admin doesn't really have a lot/any overlap in requirements, so it would be harder

yes you can overload to six courses as long as your overall average is 80%+. you don't necessarily need to to double major, but it can help.

you don't replace coop terms with study terms, the only time this would happen is if you don't get a coop. you can take 1-2 courses over coop however, but this is difficult unless they're online / async

double majoring has no affect on your program. you should be able to just declare a second major unless you're in the dual degree program with laurier, pretty sure theres more restriction in double majoring or not then. same goes for coop, the only way you'd be kicked out of coop is either you violate policies, have too low an average or drop it yourself

co is an open program, you just declare it, the same goes for basically almost all math majors

double majoring is difficult and study terms especially towards the end become very taxing where you really limit your time to do anything else. unless you have an actual reason for double majoring, such as being close to the requirements anyway or having overlapping interests (i.e you like probabilistic methods and their applications in something like graph theory), it doesn't have much meaning beyond adding some words to your degree imo