If a unit like Shen is summoned after the initial 'When I defend/attack' chain has resolved, does that unit get -1 might from Ahri's defend ability, or has Ahri's ability already passed?

Ruling: The Ahri trigger will go on the chain whenever a unit gains the attacker designation for the first time each combat, regardless of when they gain that designation.

Nuances:

• Units summoned after the initial defend/attack chain will still trigger Ahri's ability if they become attackers
• The timing of when a unit becomes an attacker doesn't matter - Ahri's ability applies to any unit that gains attacker status for the first time in that combat