For the second one, nothing stop you from publishing a demo/prototype for this game jam like all the participants.
And if you enjoy your project, spend more time to add content and polish it for a paid release in the future.

For the first one, I'm not sure but I'd say if we use game engines, you can definitely use some pre-made assets. 
But we have a month, so plenty time to replace every placeholder with cute handcrafted new assets just for the occasion X)

Just don't throw templates, plugins, assets from Unity/Unreal/any store/library and call it a game :3

I've seen both of these things done in previous Strawberry Jams, so it should be perfectly fine. There's a section for "early work" when you submit your game to the jam, so just include some information about the assets you've used. Also, for a commercial game, I suggest not locking any content behind a paywall in the jam release. Instead, add a monetized expansion after the games are rated or have the paid version be very clearly separated from the free demo. There's not any sort of rule regarding this that I know of, but it will make the rating process go smoother, since your game will be fully available for that purpose.