1) They're done (well, were) by Henson & Oz
2) Bert is essentially a non-throaty, higher version of the voice that can be lowered to be Animal, Grover, and so on (I can demonstrate this, as I pretty much have all low Oz voices down). I cannot establish a linking between the Bert voice and Fozzie when I try and duplicate them, so I can only assume that the (later--the early Foz was MUCH deeper) Fozzie voice can be linked to the Piggy voice, which might explain why I cannot do either voice.
3) Kermit is the high-end version of Ernie, which is a non-throaty, higher Rowlf. A ton of Jim and Frank voices were duplicated all the time. Even Beaker and Bunsen are beginning to sound a lot alike these days.
If you want a further explanation, I can do voice samples.