At the start of your turn while your Megatron, Megalomaniacal Mariner is on the battlefield 🠪 If one or more of your Sharkticons or Allicons were KO'd during the previous turn, you may scrap up to that many cards from your hand. For each card scrapped this way, return one of those Sharkticons or Allicons from your KO area to the battlefield and repair 1 damage from each of them. At the end of turn, KO each character returned this way.
When do you choose to add stratagems out of your sideboard? When declaring characters or when sideboarding battlecards?
Stratagems are played concurrently with the starting team for a game. This is required otherwise cards like Master Converter and Aerial Enhancements would not effect the starting team, leading Windsweeper to be sad when Blitzwing and Tracks don't count as Planes.
Does a stratagem changing the attack/defence of a character change the base values for the character?
No. For example, if Private Pteraxadon’s Binary Edgewing Scythe upgrade mode removes the base defense from a defending Windblade, Combiner Hunter, it would not remove the +1 from the Camien Toughness stratagem.
You can play up to one stratagem for each different character, trait, or faction listed on the stratagem, as long as your starting team includes that character, trait, or faction. For example, if your starting team is all Autobots and includes Optimus Prime - Legendary Warrior, you can play both Heroic Spotlight and Duty & Honor.