This is intended to be played as a pretty cut-and-dry Tempo Rogue deck that seeks to use the powerful SI:7 damage and removal effects to maintain control of the board while building your own. The inclusion of the Find the Imposter quest does muddle that a bit since you effectively lose your first turn, but the Spy Gizmo payoff is high enough to offset that. You don't have to play the quest on turn 1, obviously, and it could be beneficial to hold onto it to help you combo your SI:7 Agent, but I'd always play it on turn 1 if your mulligan included SI:7 Skulker or SI:7 Extortion.

Shroud of Concealment is in there to specifically help us get our SI:7 minions out of the deck, and is particularly powerful with SI:7 Informant. Garrote should help with chip damage to finish off your opponent, but could probably be replaced with Wicked Stab (Rank 1). The single copy of Shadowstep is in there in part to help with quest completion (e.g. turn 1 quest into turn 2 SI:7 Skulker, Shadowstep, SI:7 Skulker), but could also be great with Spymaster Scabbs if you need some extra value in the late game. It might be good enough to include two copies, but I'm not sure.

Maestra of the Masquerade is just in there to screw with the opponent's mulligan, and should always be thrown back into the deck. Particularly since this deck is running the quest, and has no other real plays on turn 1, you're not going to maintain the ruse of being another class for very long. But any chance to convince your opponent that you aren't playing a quest deck seems valuable, and starting with an alternative hero power could be useful if you're waiting to use your quest as part of a combo play. If the meta is full of quest decks then that value is probably diminished, and she could easily be cut regardless if you're lacking dust. I'd probably try that second copy of Shadowstep.