Just to clarify, is Side Step vs Crossbones’ Machine Gun damage generally defending? Similarly for Side Step…

Yes, playing Side Step as an Interrupt to Crossbones’ Machine Gun ability would make the player the defender of the incoming attack. This is a change to an old ruling we made, but we believe it’s important that an interrupt to an attack occur when the attack is initiated, but before it’s resolved—as in, the attack has started, but you haven’t resolved step 1 of it yet.

And, since you’d be playing Side Step during an attack, that makes you the defender of that attack. As for the Concussive Blast interaction, you playing a defense event within the steps of an attack will make you the defender of the attack if there isn’t one already. This occurs even though Concussive Blast’s damage isn’t “damage from an attack.”