Can Draven, Vanquisher decide whether to use his attack ability after the opponent decides whether to retreat from BFS Reaver's Row?

Ruling: Yes, Draven's "may" ability is decided when his trigger resolves, which happens after Reaver's Row resolves. The defender's decision to retreat comes first, then the attacker decides whether to pump Draven.

Sequence:

• Reaver's Row resolves and defender decides whether to retreat
• Draven's attack trigger resolves and attacker decides whether to use the "may" ability

Nuances:

• Attack/Defend abilities trigger the first time a unit is assigned as attacker or defender per combat
• Units can show up late to combat (such as being moved by Tideturner during actions) and still use their attack/defend abilities