If he was happy with a conclusion like that, he'd just have taken the first opportunity he got in that tea party. Not happening, at this point at least.
My (current) theory on "Battler's sin":
Spoiler:
Battler was the one who introduced Maria to magic. 6 years ago, when she was 3, at the family conference he believed in magic and taught some to Maria, with the promise of teaching her more about it next year.
However, with the death of Asumu, he became disillusioned with it, since magic was unable to prevent the death of his (supposed) mom, hence his current adamant rejection of magic. The whole magic period of his life was buried under the stronger impression in his mind of the tragedy, and forgotten.
But for Maria, this would have been a huge letdown when he didn't show up again. Breaking this promise could be "Battler's sin".
To extend this and make it a bit crazier, piece-Beato within the game seems unsure of whether Battler is the Battler from 6 years ago, just as Maria might be. It's possible that Maria remembers that Asumu is Battler's mom, which leads to Beato's knowledge of the same "fact".
Maria and piece-Beato probably had the intention of getting Battler, who introduced Maria to magic, to join Mariage Sorciere, but were highly disappointed to find that he didn't believe in magic anymore. Beato suspects that this may not be the same Battler as before, and questions his identity, not realizing that the previous Battler was not Asumu's child either.
That would explain how Beato can look personally offended at something that didn't happen to her, at least.
As for Asumu's Battler, who knows? Maybe we'll see a furniture called Senon eventually, haha.
I'm talking about it fairly definitively, but it's all conjecture. Pretend it's blue if you want.