I think the difference between Naolin healing Brennan and Sloane siphoning Dain's power into Brennan is one in one vs. a 3 person chain, Dain, Sloane, then Brennan. Naolin may have turned Venin to save Brennan and that made the mark, but it could also be that Sloane being just a conduit between Dain and Brennan may make all the difference. It may also mean that Sloane alone has a special power to prevent any mark. I think it's too early to say. They're different situations so we definitely need more info!