1) FATE is built on top of the old FUDGE system, which still shows in the die mechanisms. FATE 2.0 was close to those origins and therefore acknowledges them. (FATE 1.0 was never a full published game, just an early internal set of designs.)
2) Spirit of the Century was originally a standalone game set in a pulp-like world. This was the development of some of the ideas in FATE 2.0. FATE 3.0 is built of rules and ideas used in SotC, Diaspora, and Dresden Files. Dresden Files is the "most up to date" FATE 3.0 rulebook, and can be seen as the "definitive" version.
3) Because internet. There isn't an "official" FATE 3.0 SotC, but it was first published in 2006 and the FATE system has improved in several ways since then - in some ways SotC is "FATE 2.5". So several SotC fans have updated it for the newer 3.0 ruleset - each in their own way.
4) Yes, but it's also about to be superseded. The closest there is to a final "Fate 3.0" rulebook is the Dresden Files rulebook, which includes all the improvements to date.
There is a new version funded on Kickstarter, Fate Core. Fate Core is essentially content-complete (as of March 2013), but not quite sent to press yet. They're still taking last-minute corrections and suggestions, but there won't be any major gameplay changes. Fate Core is based strongly on FATE 3.0, and includes most of the gameplay improvements in Dresden Files, but has been tidied up and rewritten.