A sac outlet that produces mana seems like a recipe for disaster, so I have to imagine they were aware of the combo potential with Warren Soultrader when they printed it
And lets not forget you can always attack with Gardevoir ex itself and deal quite some damage, even though it wont be probably enough to pick the one hit KO on another ex/V Pokmon
Stealing something like an opponents commander or combo piece can absolutely shut down their game plan, though you have to be careful since it is an aggressively unfun strategy
Green gets bite spells, black burn spells like Consuming Corruption tend to gain life, and white ones like Hobbit's Sting or Joust Through relate to creatures and combat, for example
Mithril Coat grants indestructible, while Celestial Armor gives both indestructible and hexproof, shielding their user from single-target exile spells like Swords to Plowshares
The crew 1 bit is largely irrelevant, so concentrate on how close to free this makes all your equips and how free it makes them with cost reducers