Although it is subtle, I believe Vamplord Mcscurlock is no friend of the bloodletters and Oskarr would love to see him dissappear. So maybe they can ally there.
Arcy has made the change from focusing on the past to now focusing on the future, fueled by her 'cold' trauma. I can see her shrug off Marlane's death with little emotion at this point.
Oskarr has his own agenda's, as long as those are not in danger, he is down for anything. He comes from a calculated and rational point of view though. But with Arcy he has shown that, even though Arcy's plan was bad, he realized that if he wanted to continue to puppetmaster Arcy, he either had to help her with the plan to solidify their relationship or lose her as an asset in his plans.
At least that's what happened in my mind.