1. No restrictions. There have been plenty of uniforms, just not recently. Look up Adrian Butterfield in the Costume Con archives, I assure you that they were not part of the movement that uniform represents, and yet the costume won best in show.
2. I don't see a problem with wearing rank insignia. The WAC uniform is not currently used by the military. Even if it was, it's for a competition where you are understood to be wearing something that you are not. It's not like you are going out to eat in the uniform, or going anywhere else where people might assume you earned that rank. To me, it's just as offensive as actors wearing rank insignia - not at all because we know they are pretending for entertainment, not insulting.
3. Look up the uniform for question #1, then think if you need to ask that question again.
Also, a great resource for uniform questions is Byron Connell. I've been pestering him recently with a few of my own