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Memento is a great move, if your opponent switches, you get a free turn to set up Dragon Dance on your Haxorus and such!Purify Song - Seems alright. It doesn't affect sound-based moves that don't deal damage, like Supersonic. If a Pokémon with Bug Buzz gets Purify Song, it'd be pretty powerful.
Dragon Call - A Dragon-type, confusing Scald. Solid move.
Cotton Spin - Given that Cotton Spin has the same distribution as Cotton Guard, I'd say that the magnitude of the Attack drop is fine.
Two Attack, Sp. Atk, and Speed boosts + Baton Pass in one move, without making the user faint, would be broken. A net +6 stat increase without a switch to a Pokémon that could abuse those stat boosts is already extremely broken. Being able to switch would make that even more broken.
The reason why Memento doesn't see much play at all is because the affected foe can just switch. I get that a Dark Linked Pokémon is much harder to get rid of, since they can't just be eliminated by the opponent switching out or anything; the foe would have to use Roar (but survive the Dark Linked Pokémon's attack, first). If Speed weren't increased, then at least the foe would still be able to faint the Dark Linked Pokémon before it goes on its tirade.
Saying that increasing offensive stats by two stages each being broken is totally fine, but to say that losing the user is meaningless isn't quite accurate. If it were meaningless, then having the user faint for some effect wouldn't be a penalty for moves like Healing Wish or Memento in the first place.