1. Rubber band. I have them too, but most of the other people I know don't. I didn't start using them until I started doing more advanced releases (I think it was working the catch and kip out of a pak that did it) and my finger holes were getting kind of bigger than I would have liked but I wanted to keep the
grips. I hook two rubber bands through my finger holes (I have a weird way of doing it) and then I slide it over the part with holes and hook the buckle. It's hard to explain but it's not really necessary. I wouldn't worry about it unless you start to have problems.
2. I'm not sure I really understand? The dowel is attached to the grip right under the finger holes.
3. I wouldn't like those. Most people seem to take them off when they're attached to the dowel but I have been older since those started popping up and the US Glove Hot Shot
grips kind of started it, so maybe it helps for little ones, don't really know. It's there for the same reason as the first picture really, to keep fingers from slipping out of the holes, but I don't like how it's attached to the dowel. I attach mine through the finger holes so it's more on top. You could just cut them off if you don't like them.
4. I don't really know. What kind do your teammates have? I would just what the gym sell honestly unless people seem to not like it or something. Some coaches have dowel requirements, etc, and they can help you choose something good and size them usually.