Hi Lauren,
Love that you are planning ahead! It will make your trip go a lot smoother and leaves you more time for fun instead of trying to work out the details.
You are more than welcome to bring your backpack in the line. In fact, you should not leave any of your valuables in the stroller and should bring them with you. I've never had any problems with leaving things in my stroller but you do so at your own risk and Disney is not responsible for any items you leave behind.
In your backpack, bring something to help keep your 9 month old entertained in line - favorite small toys, snacks, bubbles, etc. To help pass the time, you can also play games like peek-a-boo.
Be sure to give your 9 month old breaks from his/her stroller. Try (it will be hard) to stick to nap times. Go to the parks early while your child is fresh. When you see he/she has had enough, pack it up and head back to your resort.
Click
here for some tips on strollers.
Wishing you minnie magical memories :-) Allison