If the monster somehow leaves the field, or is switched into defence mode, before the Damage Step can start, then the attack just ends. That usually happens when cards like Sakuretsu Armor or Enemy Controller are used, but there are many others, including De-Fusion on a Fusion Monster. So De-Fusion will end the attack in the same way as Sakuretsu Armor does. The defending monster will not be flipped if face-down, and no damage will be calculated.
'Negated' though is the wrong term, that's something else, something different and specific. However the end result is mostly the same.
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Yes, De-Fusion is Quickplay so you can attack with the Fusion Monster, then use De-Fusion on it, and since it's still the battle phase, the newly summoned components can attack too.
if you attack with a move it counts as 2 pp points. imagine if the opponents Pokemon has the effect pressure, and you use tackle and tackles pp was 10, then after you use tackle it's pp would be 8
no, it's a normal magic card, you can only use it when it's your turn.
ah, my specialty. Well, that depends. Does your opponent use compulsory evacuation device (i think) when you attack them? Because then the attack is negated. If a player controls one monster, and this monster can't be selected as an attack target, then you may attack directly. So one situation will be if the player has Umi and The Legendary Fisherman, or Guardian Kay'est. These can be ignored, and a direct attack can be made. However - this does not apply if the one monster is an Earthbound Immortal. If a player only controls one monster, and this is an Earthbound Immortal, then he cannot be attacked in the same way, the monster cannot be bypassed, monsters will simply be unable to attack. This is a specifically ruled difference, though without explanation from Konami. If the player controls two unattackable monsters, then again, player and monsters are safe from being attacked. No attacks can be declared against any of them unless circumstances change.
Yes you can if you use the card "double attack."