You knew this was coming:
1) Can you fight another PC? The text reads like you can.
2) Does the GM roll too? If so, does the GM have access to all the moves, or just fighting? If not, who rolls when your enemy is an NPC?
3) How does this interact with interrupting someone?
On 10+, they just stop your attack?
On 7-9, you have to back down or fight back (ie, you're still fighting them) -- in which case you both roll fight with someone now?
[edited my pronouns]