Yes for most.. its V3 of the engine with certain updates.
No, its the same basic code.
A little.. its more about new features like flame weapons, hit decals, AA fire on aircraft and water animation showing when units ford streams.
Definately playable as is but there are a few minor issues that will be sorted with the 1st patch. Might want to wait and try the demo for the respective installments if you are uncertain.
Yes it will. Estimated two months after RT release earlier by Steve. So june, july or somesuch. Note that it, just like 2.0, will be a upgrade and not a free patch.