Hey there,
1. Yes external assets are fine to use but we ask you have the proper rights to them and credit them on your entry.
2. You can source textures and pre-made assets, write an outline and such, however, we ask you do not include your own models, levels, code, etc before the jam period.
3. We discourage just submitting a game or complete parts of a game you worked on in the past, and we certainly encourage you to make your assets during the jam period, however, we understand that many solo devs make use of texture libraries, and other free or bought assets.
In the future we'll try and be more specific regarding certain rules. Apologies for any confusion, and if you have any more questions please let us know!