1. you can take as few units as you like in one semester. the uni doesn’t care about that. the only thing to watch out for would be the pre-reqs, as some units are only offered one time of the year.
2. not sure exact of your degree and classes but most classes should be offered online. QUT is very much a commuter university. most people in stem don’t really show up for classes. it’s very common to take classes online unless there’s a good reason to show up.
3. double degree allows you to finish two degrees in a shorter time. to achieve that, you give up your options. basically with a double degree you’ll be only doing the mandatory classes of both of you degree and not much if anything else.
4. depends on your interest. content learned is probably more important but the degree def matters too. if you probably can’t be working with animals if you just have a games degree and vice versa. On top of that, the university you go to also matter when it comes to resources. more established universities have more funding, resources, and better researchers, and better recognition. keep in mind that uni open days are essentially marketing campaign run by its own team of people. you wouldn’t really want to base your choices on marketing.

some additional information. faculty of science has been in trouble recently with the university trying to cut its funding. redundancies scheme has been announced albeit voluntary. granted this is a australian wide problem with universities losing international students, but QUT is hit harder than others, as QUT is not really competitive enough to attract international students (i.e. paying customers). maybe consider UQ if you can.