As far as I understand, the closing period is just for what will judged in this Jam. That version of the file will be locked down from changes but nothing is stopping you from keeping the project going.
I got some tips for you on this one! I actually made a very similar game for my very first project. A cheat to keep your character looking good on the shooting scenes, use a sprite for the gun that is separate from your main sprite. The gun will rotate around a stationary person and it will make things like making a walking animation much easier.
The other tip is more broad but one that I think a lot of people could find some benefit in: Go find a game that plays like the one you want to make, and "steal" from it. As long as you aren't literally copying and pasting from someone, use their ideas and do your best to replicate them. That is basically what all art is, just remixing everyone else's ideas in a new package. It can give you some clear direction on where to go instead of doing the classic "implement new feature instead of making real progress" thing.