Well, that whole "which partners would you have sent" doesn't really ring true since they don't seem to know that the stone demands a soul sacrifice. Nebula seems to have left that piece of knowledge out? Because if they did, surely it would have informed their discussions. (Chalk that one up to a plot hole...)

All this to say, I am working on a fixit for that whole soul stone thing, because it doesn't make sense in many other ways.