If I have 2 units at a battlefield but one is recalled or killed after my opponent moves in and starts a showdown, does my remaining unit get the 'when I defend alone' bonus?

Ruling: No, "when I defend alone" bonuses do not trigger in this situation. These abilities only trigger when a unit gains the defender designation, and at that moment the unit was not alone.

Nuances:

• "While I defend alone" passives (like those on Wielder of Water or Master Yi) would activate, as these are not timing-based and update in real time based on the current battlefield situation
• The distinction is between "when" triggers (timing-based, check conditions at moment of gaining defender status) and "while" passives (continuous, check current state)