Yes, it does, if you don't complete Legion's loyalty mission on Mass Effect 2 he won't survive The Suicide Mission, therefore he won't be in Mass Effect 3 and he is replaced by a AI of Legion.
Yes It is possible but you have to have a paragon to be able to save her
Yes, there will be a Mass effect 3. even if your Shepard died, there will be a mass effect 3.
It just effects some interactions and choices in the game.
Mass Effect 3 will be released March 6, 2012
Mass Effect 3 will be released March 9, 2012
You do not need to own ME2 to play ME3, but if you have ME2 you can port you save into ME3, which causes choises you made in ME2 to affect the story in ME3
yes
If you played Mass Effect 1 the second game imports your old game save, so it would depend on how you played Mass Effect 1 as to whether or not Kaiden is alive. If you did not play Mass Effect 1, then the game will default to the character with the opposite gender to Shepard. So Ashley will be there with a male Shepard, Kaiden for a female one. Below is a link which has the important decisions you make in 1 and their effect in the sequel.
You can scan planets and build relationships with the characters. You can also import the save game into Mass Effect 3, or import it into Mass Effect 2 so you start at the same level as when you finished the game. Other bonuses are included if you choose that option.
The SSV Normandy in Mass Effect The Normandy SR-2 in Mass Effect 2 & 3
Can the PC play the Xbox 360 game Disc. No of course not. They are not the same game and only have the same title. You don't even have the same title and want to save a different game in the series on a platform it will not play on