I play around with hop extract quite a bit, so I can probably lend some guidance here.
A. Hop extracts (aka hop shots) are typically CTZ. They aren't always though. Apollo, Bravo, Summit, and any number of the other Super-Alpha hops make their way into hop extract. CTZ has the most acreage, so it tends to be the most commonly used.
B. Yes, it definitely can be used for late additions. All the extract I've procured has smelled just like CTZ.
C. Theoretically, you could use it to breath some life into an old beer, but I think the problem would be from an operational sense. Hop extract is really thick, and does not like to dissolve in cold liquid. If you could find a way to get it to dissolve into cold beer/wort, ya, it would work fine.
D. They are starting to make all sorts of variety specific extract. Vinnie has mentioned that Blind Pig now uses Amarillo extract at the 30 min addition. I'm still kind of a purist in that sense. I'll continue to use pellets or leafs for flavor and aroma.