If "Infernal Dragon" attacks, and is then flipped face-down, it will not be destroyed during the End Phase. Even if it is flipped face-up again during the same turn, it will not be destroyed.
If "Infernal Dragon" attacks, and control of it is switched to the opponent that turn, it will still be destroyed during the End Phase.
Even if "Infernal Dragon's" attack is negated by "Magic Cylinder", etc., it will still be destroyed during the End Phase.
The second effect of "Infernal Dragon" activates in the Graveyard. If it is destroyed while your opponent controls it, the original owner can choose to activate this effect.
The second effect of "Infernal Dragon" can be activated even if it was treated as an Equip Card when destroyed.