I'm doing one of these, so I can help you out a bit (I hope!). Note that the exact details are different between each diploma so what I say may not quite be the gospel truth.
Q1/2. With regards to cross-crediting, this means 4 of those maths subjects can be credited to both your degree and the diploma itself; if you cross-credit two subjects, your biomedicine degree will then have a remaining 275 points, and your diploma 75 (the diploma will consist of 100 points (eight mathematics subjects) from a certain list of choices; the handbook entry should help with that). The diploma works effectively like an extension to your course, but means you will most likely be spending a fourth year at university (overloading and cross-crediting can mean that completion in three years is possible, but you will need to overload!). Once your application is accepted the diploma comes up as a separate course in your study plan tab on the portal, and you can organise it independent of your biomedicine degree. Basically, so long as you get the overall 400 points done within four years the rest is up to you and the student centre probably won't even be involved. This also adds some flexibility to your degree if you cross-credit; the "empty spaces" opened up in your four-year study plan mean you have some wriggle room for where you put your various subjects.
Q3. Yes, the diploma costs extra money, but students are often eligible to receive the final 50 points HECS-free; no debt is incurred if you're in a Commonwealth Supported Place and so you have four effectively free subjects.
Q4. I have no idea, you'd have to inquire about that yourself. However, as the diploma is "attached" to your main course I'd assume it does?
Q5. It ought to be, as the subjects don't impinge on any of your core subjects or breadths if you don't cross-credit (if you cross-credit, I'm assuming the credit is counted for science electives, meaning you probably only have the ability to cross-credit 25 points as maths subjects are not biomedicine breadth, only science selectives) so you can simply take the full 100 points independent of the biomedicine degree.
Hopefully that helped and wasn't too poorly worded (it doesn't make any sense to me, so hopefully you can pick up on it a bit better

)!
EDIT: DAMNIT AURELIAN YOU NINJA
