1. Kind of, minor bugfixes are generally accepted if you can prove that they impact the experience in a major way. Please make sure you mark future uploads as FIX or 1.1 or similar to show that it is not the vanilla upload.
2. No. No tricks. A dummy is fine, but don't submit something that is not finished yet. Update your dummy and then submit it to the jam when you're done.
Bonus. HTML5 covers all platforms, totally fine substitute.