I think Klaus is happy to see Othar because Boris and Chakraborty will attempt to follow established protocol, Trelawney will at least try non-violent methods first, and Martellus might consider the political implications before attacking, but Othar is just trying to take Klaus down, and that's the most likely thing to prevent Klaus from doing whatever Lucrezia ordered him to do. (Which is probably growing her a new body and copying her personality into it. Which would be Bad. Protecting the tank from Othar is legitimately following orders without moving the plan forward.)