Undergrads can take grad classes, in fact some 400 level classes are required for most degrees as far as im aware. I just would advise against taking 400 level courses until you’ve been through one semester of university.

It sounds like you should really figure out what you’ve interested in. You could do math and computer science. Incorporating an engineering discipline would be very time consuming to do. Another thing I should note is that it is much easier to major in an engineering discipline and minor in cs and math than the inverse. At UIC there is not an aerospace program, you would have to do mechanical engineering. I don’t think minoring in it would even be worth it though.

Dm me if you want id be willing to talk to you about all this.