In this scenario, the Stratagem
Sky Shadow Sync is also in play. What exactly happens when you only have Sky Shadow (Tank) and Sky Shadow (Plane) as your last characters and both get
KO'd at the same time?
[Source: WotC]
If all your characters are
KOed, and there is no other card text saying otherwise (like a head's deploy ability), you lose.
Sky Shadow Sync doesn't include such text, so if Sky Shadow (Tank) and Sky Shadow (Plane) are your last characters and both get KO'd at the same time, you lose the game.
If I attack one part of an uncombined Sky Shadow, and a triggered ability KO's that part before flipping cards resulting in Sky Shadow combining, does the attack end? Or does the attack continue against the combined Sky Shadow?
[Source: WotC]
The attack continues since "Combiner Mode" is mostly another mode like Alt or Bot mode. You just can't "flip" to combiner mode without another card telling you to. Going to combiner mode is a one-way operation. Note that Sky Shadow will be untapped because the process of combining creates an untapped character.
As Sky Shadow does 3 damage to an Autobot when it combines, this is a very relevant question. The answer is that you resolve the card's contents first, and then resolve any effects that triggered in reverse order. This means that you flip
Metroplex to alt mode, triggering his deploy effect, then you move the damage to KO the Sky Shadow piece. This event triggers a new effect, the combining of Sky Shadow, and the Sky Shadow player chooses an Autobot to do 3 damage to. Lastly, you return to your stack of triggered effects and deploy your Metroplex minion.