1. No idea.
2. No, afaik also used in CS/Quake-clan type circles. Unlikely it's unique since the term is wide spread on google (with the spelling of l33t tho). There are numerous domains registered involving the word to.
3. (i) Someone of the "elite" - best of the best, player with skillz. The opposite of a n00b.
(ii) Something very good or cool, having attributes that are highly desirable, etc. "The new Radeon Ultra Bollox is leet", etc.
(iii) leet-speek - a dialect used by on-line communities. Originally probably partially derived from phreaker and hacker speak. On some warez BBSs (back in the day) eVeRY oNe TYPeD LiKE THiS or some variation (I think that was warez speak, I forget, I only tried it once to get onto the 4th Reich (pro-NAZI warez bbs for some reason...) when 13 and the bloke said "Ah bollocks to it", in the middle of the conversation and started typing normally).
Basically l33t speak was used by the warez types to differentiate themselves from n00bs. Unlike nowdays there were no Peer-to-peer networks, you needed to be given access to someones BBS site. So everything depended on how leet you were. This community still exists to a certain extent until recently with news from
www.isonews.com. I've no idea how many of the groups still exist (Razor 1911, etc, etc) and if they still type like morons.
I believe the original point to a lot of this nonsense was that Sysadmins would search for "Software" but not "warez". Possibly.
"Modern" leet speak seems to have dropped most of the cap-deforming actions but still pisses about with everyone second word. There are numerous leet translators on the web.
4. l33t or leet I'd say.
All my humble opinion, mostly wrong, etc, etc.