Babies are born face down because the crown of the head comes out first so it would make more sence when they come out.
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You can attack your opponent's face-down monsters. You cannot attack his face-down S/T cards though.
No, face-down monsters are valid tributes, you are not forced to flip them first. There might be one or two cards that do require the tributes to be face up, but these will specify in the effect text.
Yes, the tributed monster does not have to be face-up at all, it can be tributed while face-down.
Depends on the scanner, but generally: if you load it onto the glass, face down if you load it into a hopper/feeder, face up
A monster in face-down Attack position is unable to declare attacks. It can be flipped or Flip Summoned, both actions will switch it into face-up Attack position. It can also have its battle position change by the controlling player, switching it from face-down Attack position to face-down Defence position.