1. The unit is basically full on opening from the box (samples/system data/.wavs...) deleting the wavs will free up a lot of space, and also the samples will. But don't delete samples you don't want. ie, i KNOW there a samples on there I will never use! Cannot extend memory but future algorithm will be able to compress samples more without loss of quality.
2. wav only at the moment, I don't know if mp3 functionality will be added later. Although you can attach an external player.
3. Not sure about this, I assume you change an existing kit with sounds you want. Again from my point of view some will NEVER be used by me....
Hope that helps.