Book 4 spoilers
One more thing about Benno I came across when browsing through the books. There is one scene in Book 4 where Myne collapses after spending too much mana and skipping a meal and he is offering her a lap pillow and holding her hand. Mark even has to remind him that she’s not Liz, his fiancée, and won’t necessarily end up as her. So I think there are quite compelling reasons to believe he’s attached to her due to motives beyond making profit.