1. Can I modify the 3D assets?

To an extent you can, you can remove parts of the model, you can change their shape using shaders or change their color and add effects onto them using shaders aswell. You cannot put them in a 3D editor and change their shape, the only exception to editing the model directly will be that of when you want to remove parts of the model or split it in different sections.

2. Can I use my own UI assets?

Previously the answer was no, but primitive UI can be super boring. So the answer is now yes you can as long as those UI assets are used for UI only and not for gameplay elements like characters, bullets, vehicles and so on. It has to be strictly used for user interface. So audio sliders, menu buttons, health bars, score counters etc.

3. Can I only use the given toolkit sounds?

No, you can use any sounds you like. You can make your own, change the given ones or download some from the internet. The toolkit sounds are only an extra to help!

4. Can I make a 2D game?

Yes but then you would have to use the models in an orthographic view or use primitives like squares and circles to make the game as you cannot use custom UI elements to build gameplay features.

5. Do I have to use the toolkit?

No, if you wish you can create an entire game out of primitives. But keep in mind that in the rating period there will be a "toolkit usage" category.