Well, I would have to also ask the application for the firearm before I could answer number 3.
1.- This is a type of quick acquisition sight that has become very popular, especially with the shotgun for home defense crowd. The rear sight is a lighted/light retaining/iridium ring that is quite thin. The front post sight is thicker, and also has a vertical vial of the same lighted/light retaining type stuff.
The sales pitch is that the thick front post is put on the target easily and the thin rear ring is easy to line up, thus making finding your target quicker.
I like them, but I don't have a set on my shotgun because I don't use it for home defense or anything like that.
2. - Complete loss. I have no idea.