You cannot Special Summon "Montage Dragon" while "Macro Cosmos" or "Dimensional Fissure", etc. is face-up on the field because you cannot send Monster Cards from your hand to the GY.
If you send a WATER monster from your hand to the GY while "A Legendary Ocean" is active, the original Level of the monster is used and not the reduced Level.
If "Montage Dragon" is flipped face-down by "Book of Moon", etc., when it is flipped face-up its ATK is 0.
If "Skill Drain" is active while "Montage Dragon" is face-up on the field, its ATK becomes 0. If "Skill Drain" is later negated or removed from the field, its ATK remains 0.