2) Is constant Origin connection required or is it a single one off authentication when the game is first installed. Is there also a limit to the number of installations available? Mass Effect 3 will require a one time, single authorization for the single player game. There is no limit to the number of installs. Playing Mass Effect 3 Multiplayer will require a constant connection.
Amazon makes their policy with game downloads very clear. If there is some type of DRM that limits the number of installs, and you run out of installs, Amazon will send you another code for the game, free of charge.
From right here on Amazon:
"Can I download and install to multiple computers? For video games: Each game manufacturer has a different policy on how many installations are allowed with each product key. If you run out of installations for the product key you originally purchased from Amazon, please contact Customer Service and we will happily provide you with another key at no additional charge. You can install an unlimited number of times for personal use, however additional copies of the game for friends or family must be purchased separately."
So just for future reference, even if a game does limit you, purchasing from Amazon will essentially give you unlimited installs.