Zone.com is not bad, but there are alot of cheaters/hackers around, and a lot of lame players to contend with.
as for your questions:
1 & 2) Massassi.net, first get patch commander (your cue, Kurgan
; the link is?)
3)I don't know
4)Rbots can be found through a link at Massassi.net.
as for popular levels and mods:
Jedi High School appears to be the most popular level fo JK on the zone, mostly used with the Killers Weapons Pack, a collection of over powered guns, with a nice grapple mod thrown in. Also try Morphous, a level i love, but never came out of a MP game with a positive score!!
Get BFP2X from JKmag- also popular (i like the MechaJedi skin myself!)
Other popular mods include Spork 1.2 and the Spork level pack.
As for MoTS, the only mod I have seen used regularly is Manowars2, also at JKmag.
hope this helps-
The Master is Back
with a vengence