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VSing DAD decks, Shien IS enough. They don't usually run monster removal in the form of magic cards so halting them down to 1 M/T per turn while maintaining normal speed is better then slowing down your magics.
I don't necessarily think Anti Spell Fragrance is needed; it slows down the deck a fair bit. It gives a good opportunity for Yaichi to target their M/T, but overall, Shien works better then Anti Spell Fragrance.
Not if the Pokemon is fully evolved, so in your situation, the Breloom would keep Effect Spore. If you had a Effect Spore Shroomish in Emerald and transfered it to DP, there's a chance of the ability changing when it evolves.
Smeargle can certainly Sketch it's own teammates move. Just make sure it's a double battle in the game and not on Wifi or against your friend.
Sketch will go depending on how fast your Smeargle is, just like any other move.