Here is a brief rundown.

1. If you are going to Botcon from long distance, then pre-reg is a must.
You are able to do everything, and not be in the long queue.
I didn't take a pic, but the queue on Sunday morning for general admission was INSANE.

2. No. Even going to Botcon, it costs more to join up than you save on your pre-reg.

3. If you want to go for more than just Botcon, add Wednesday & Thursday as they run tours of the area. I don't feel it is worth an international flight just for Botcon, so if you can, make a holiday of it.

4. Pre-reg, they take your $$ as soon as they can.

Hope that helps.