Just want a quick clarification for my own sanity.

Is there an opportunity to trigger actions (not just responses) after declaring a character as a defender during an enemy attack but before resolving the combat damage?

Yes, you can take actions at the end of all 4 steps in combat (Choose An Enemy, Declare Defender, Resolve Shadow Effect, Determine Damage). So you can take actions after exhausting a defender but before revealing the shadow card, and you can take actions after revealing and resolving the shadow card but before dealing damage.