When does combat start when moving a unit to an occupied battlefield, and does Draven's legend trigger if the opponent's unit is removed by Gust in response to a 'when I defend' effect?

Ruling: Combat is initiated when there is a staged combat at the battlefield and the turn is in a neutral open state. In this scenario, combat starts after the Merchant's move effect resolves, so Gust is played after combat has already started, meaning Draven's legend will trigger for winning combat.

Sequence:

• Contested is applied when the Merchant moves to the battlefield (step 1)
• Combat is staged at this point
• After the Merchant's move effect resolves (step 2), the staged combat is initiated and the initial chain is populated with the defend trigger
• Playing Gust in response to the defend trigger happens after combat has already started
• Barring your unit being removed or an enemy unit moving to the battlefield, you will win this combat by default

Nuances:

• A battlefield becomes contested when a unit becomes located at a battlefield not controlled by its controller
• Combat is staged at a battlefield when a battlefield is contested and there are units controlled by different players there