Only the Spell card Darkness Approaches can flip a monster into face-down Attack position. By discarding two cards, Darkness Approaches lets you flip one face-up monster face-down without changing the battle position (so a monster may end up in face-down Attack position).
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A monster can't manually change battle position on the turn it declared an attack, but it can still be flipped face-down by an effect. Golem Sentry can attack, then activate its effect in main phase 2 and flip itself face down.
Assuming the defending monster is also in attack position, both monsters will be destroyed.
Yes. If your monster is in attack position and you attack a monster that has more attack points than yours, your monster is destroyed and you take Battle Damage to your Life Points equal to the difference between your opponent's monster's Attack Points and your monster's Attack Points.
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.
Your monster is destroyed if the monster you attack is in attack position, and you lose life points.