1. Or local multiplayer (Ad-hoc) or via a server on the Internet (Infrastructure). But the first time it could be a server developer, not PSN, AFAIK.
2. The simple answer is "Not only". It depends on the game at least.
3. Since then, and we had to start. At the moment, no. I think it will take more than enough time to fully break-in. Let the first step is already done (so far only in words).
4. At least the ability to install plugins (services running in the background and adding different functions like taking screenshots).
5. PSP for sure, not all games came out on PSN. As of now I do not know.
6. Yes. Yet there can be about game sharing still remember. He allowed including local multiplayer with a single UMD. Ad hoc from one PSP to the other is transmitted to only the necessary part of the game and only for the duration of the game. But not many developers bother to support this chip.
8. Worms, Tekken, Sonic, Bomberman, for example. I can not vouch that all have multiplayer on the Internet.
PS. PS Vita not PSP Vita.