So the Momon and Ainz scene has an explanation, and they've talked about it before. When the workers were invading Nazrick, Momon was guarding their camp. Ainz went back to Nazrick to watch and join what was going on, and he mentioned that Pandora's Actor took his place as Momon. Recall that Pandora's Actor (The npc that Ainz had made) is an extremely powerful shape shifter and is an equal to the other guardians. We can reasonable assume that was him here.
Also I think you missed when Momon said "No one will marry a woman like you" to albedo. Hilarious under the context that the person who is sort of like Ainz' "son" said it. I dont think PA and Albedo have the best relationship lol