1. Yes that size is good for a single wire. There is a slightly larger one that I use for two wires through a single hole at locations like the front fender.

2. A second grommet would probably work, but I have the leather washers on hand. I am not sure why the Secula is designed in a way that leaves it sitting funny when mounted. It is one of those subtle things that most people would not even see or care that much about.