Legendary Marth Question

Just visible buffs I think.

The buffs must be visible to be doubled.

I'm not sure why your second question is there because Defiant skills are field buffs (visible), but field buffs are the ones that get doubled, so spurs and drives won't be doubled. The bonus is also only active in combat, but this really only matters for skills that activate before battle, like Growing and Blazing Wind, so you won't find them on many PvE enemies, or on many players.