Hi,
1. Axis isn't locked, you're free to do what you want. Nothing imposed on cameras. You can use ortho or perspective cameras too.
2. You don't have explicit control over this, you will have to be clever with z depth. If you want multiple sort layers, you can use multiple cameras with different "depth" values and different layer masks to get this control.
3. Yup, it does all of that - one thing you didn't mention is that 2D Toolkit also rotates sprites to try to get best fit. 2D Toolkit also will generate multiple atlases from your textures (atlas spanning), but I strongly recommend NOT using this feature however tempting that may be if you have large numbers of sprites. It isn't as efficient as manually doing it yourself. Animations are free to span atlases, so no issue there.
4. It uses Unity dynamic batching, so you get whatever performance boost with that. Having said that, there's also a built in Static Sprite batcher, which lets you batch static sprites (backgrounds and such) in the editor.
Hope that answers your questions, Feel free to ask if you have any more.