If I have Teemo and Nocturne in Riftbound Fields as defender, and opponent moves in with Merchant, can I flip Teemo in reaction to Reavers Row trigger and move Nocturne back?

Ruling: Yes, you can flip Teemo in reaction to Reavers Row and move Nocturne back. Once combat starts with you as controller/defender, it's under your control until the end of that combat, so you could even move Nocturne back and flip Teemo later in the showdown.

Sequence:

• Merchant triggers on move (before showdown)
• Showdown starts
• Any "when I attack" triggers go on the stack
• Reavers Row and any other "when I defend" triggers go on the stack
• Once priority comes to you, you can play Teemo with reaction
• Or you can wait for that chain to focus and then play Teemo