Although it was still only a guess, verifying his idea would not be difficult. Once he returned, he only needed to "study" Miss "Plant's" essence and see whether it shared the same "origin" as the "branches" on the "Father of Succubi" before him.
As for why Derrick had handed the Blood Moon Queen's summoning item to Gluttony, Abner lacked the clues to gain Insight into it for now, but he had roughly three guesses.
First, Derrick might have done it on a whim, without any clear purpose, just like how he had conveniently saved that Baron Iris before...
But that was highly unlikely. Saving someone could be done on a whim, but why would Derrick let the other party take Heaven's Feast away?
Second, Gluttony might have been a pawn of some existence, and Derrick had used this method to make a transaction with that existence—one Abner did not yet know about...
Third, and most likely, Heaven's Feast contained some arrangement related to Alvin. Derrick had only given it to Gluttony after discovering it, using him to destroy it.
Heaven's Feast and Alvin's Palace were respectively an item of the Gourmet Pathway and a creation of the Collector Pathway. Both were connected to the Outer Deity Primordial Hunger. After Alvin discovered the Heaven's Feast once summoned by Olynnia, the Blood Moon Queen, in that second-floor room, there was no guarantee he had not tampered with it through those connections.
After all, Abner had also felt that fusing Alvin's imprint earlier had been far too easy. Even if Alvin truly had some scheme in mind, it had been far too simplistic overall.
One had to know that Alvin had not been killed like Reinhardt and Vincent, leaving him without time to arrange many methods. He had voluntarily entered the goddess's Nation