I'm not too sure about your first question, so I'll leave it to somebody who's actually majoring in Bioengineering to answer that.
If you're in Biomed, you don't have a choice on what maths subjects you can pick unless you do a concurrent diploma. Normal 1st year biomeds are forced to do stats and Maths for Biomed since they're 1st year cores, but if you choose the Bioengineering stream you have to do Calculus 2 and Linear Algebra instead. And nope, the type of subject you pick does not influence your GPA (for MD admissions that is - for other GPA systems, some subjects might be weighted more than others but that depends on how many credit points they're worth).
I don't think your major is tremendously crucial to your career choices - but it would be pretty important for things like Honours years, etc. If you apply for post-graduate health science courses it almost makes zilch of an impact.
And no, your breadths are entirely picked by you alone. But a breadth is a subject that is outside of your course, i.e, an arts subject if you're doing a science/biomed degree. So it won't relate to the major you're thinking of doing. You only need to do a minimum of 4 breadth subjects over your degree, and only a maximum of 3 of your breadths can be level 1.