Different faction means "does not share a faction identity." For a character which is both an Autobot and a Decepticon, "not sharing a faction identity" means NOT Autobot and NOT Decepticon. Currently, this leaves Mercenary as the only different faction.
Card requirements are inclusive, not exclusive. However, in this case, Cerebros' Head is not active on the battlefield, so it doesn't contribute its faction identity or traits to the character. So, a Decepticon Head of Cerebros doesn't let you play a Scoundrel's Blaster on Fortress Maximus.
Card requirements are inclusive, not exclusive. This character fulfills both the AUTOBOT and LEADER requirements on Matrix of Leadership, so you may play the card on the character and the 'While on a Leader" ability will function.
Cards try to do as much as they can. You can play Unconventional Flying Object even if you don't have a Spaceship and it will only accomplish putting the Action into your scrap. There are a handful of Upgrades like Crushing Treads that are restricted to play on say "Put only on a Tank." – you can't play that card unless you have the specific kind of character to put it on. After you successfully put it on the right kind of character, the game won't check anymore if the upgrade is valid, so the character can flip to Bot mode, stop being a Tank, and the Upgrade will stay on.
A head’s deploy ability is a triggered ability. When both players trigger an ability at the same time the active player’s abilities will complete before the defending player. A player may order their own simultaneous triggers how they wish. The player controlling Quake may order Quake’s damage trigger and Quake’s head’s deploy trigger such that Quake’s damage ability resolves first, and Quake’s head second. If Octone’s controller is the active player, they must use Octone’s bounty trigger before the defending player resolves their head’s deploy trigger.
This answer is an update on the originally published answer in the Roundup
The trigger for all "when this attacks/defends/battles" is at the same time and happens only once as the attacker is declared and tapped. If you use a "when this battles" trigger on a character to then play a "when this battles" upgrade onto that character, the upgrade won't trigger as it's now too late in the attack for those effects to trigger.
Crystal of Power will stay on. The time when Crystal of Power checks for "When the upgraded character battles" was passed the moment Soundblaster tapped to attack. Crystal of Power will trigger the next time the character battles, either defending or attacking.
A team of characters combining isn't actually untapping; the resulting Combiner character is entering the battlefield untapped (like all characters which enter the battlefield after the start of the game, unless another card or ability says otherwise).
While Fortress Maximus is in play, he and Cerebros's stars make up the character's star count. The second Head is not counted.
You can't play Fortress Maximus without also playing Cerebros, and vice versa. Since you can't put both in your sideboard because you're only allowed one character in your sideboard, you can't sideboard Fortress Maximus in any effective capacity.
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.
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.
