Write your own statement and get your doctor to sign it / write it out and print it on their letterhead
VTAC only let you submit one afaik and I don't see the value in more
For 5) in theory yes, in practice it's a lot more murky. Depends a lot (a lot) on what you actually get I would think.
For 6) Monash don't consider SEAS for UMAT at all, they might for for ATAR but that depends (this was correct 3 years ago, not 100% sure on now)

VTAC say no, you'll have to trust them on that
Your doctor won't be able to see you VTAC app, they just write the supporting doc
Submit the scholarship app, there's no reason not to. You probably won't get one, but hey, any chance is better than none
12) email VTAC and ask, or ask if you can attach a doc/send a physical letter