you can always snipe, hit the mortars and flame throwers
yes
Forgot that the Waltar has access to Lore of Light (I never play waltar), so yes, probably best to go with Shadow.
GW would work on the Knights, 'cause, yeah, rats are gonna strike first, and since most are only S3, the 2+ save will be the same (just stay away from the Plague Monks and Stormvermin)
As for a lone wizard, Skaven are known for thier trickery, and would just love to catch a lone wizard with a spell or war machine. I like to keep mine in a flesh bunker.
The only other thing I see is only 14 flaggies. If you are going to go with these guys, go big. Full 30. Yes, they are Unbreakable, but they are very easy to kill, and 14 is just not enough. Maybe want to switch to Greatswords, who tend to hang around a little longer (if you are still only going to go with 14, that is).