1) Release is supposed to be March 6 if I am correct.
2) All ME2 saves carry over to ME3. If you didn't play ME1/2 first, you're doing it wrong.
3) In ME2 you could change your character's looks in the beginning, but not undergo a sex change. So if your character in ME1/2 was female, it will likely be female only in ME3. Likewise, if your character was male if will be male only in ME3. I have two profiles with of each sex as it opens some new dialogue options.
5) The co-op affects the ME3 story, but you don't play as Shepard and the missions are co-op specific.
My female Shepard from ME1/2:


I am not sure which profile I will play first in ME3.