"Momo escaped"
"Oh no! Well anyways let's leave the queen's head somewhere where we wont see Momo kill her"
I feel like it's too brazen for Momo to come back now and try to do that when there's multiple guardians in the area that can drop her, especially Leigh Mary. Momo knows the guardians will try to kill her this time for deceiving them multiple times.
or worse, take her as bargain chips
This is more likely. I can see Momo negotiating Leigh Mary's life for the Empress' life if she actually steals the Empress' head. Best time for Momo to do this is when everyone is passed out from the drunken orgy but if she botch the heist, Momo will most likely die/ be incapacitated for good.
However, this is just messy storytelling if Momo does this during the orgy because after the orgy, MC is supposed to go a secluded village next to liberate it from a witch. He shouldn't be distracted dealing with Momo immediately after the orgy. For better story pacing, Momo should attempt to steal the empress' head while the MC is at that village and there are fewer guardians for Momo to deal with.