You have a much better chance of STBX being receptive to hearing things from the C if you don't coerce him into going. Take S to the C. Let the C tell STBX that he really needs to come in. If you want STBX to be able to hear anything from anyone, the less directly connected it is to you, the better.
Again, these things shouldn't even be on the table for s10. The custody schedule is set, stick to it.
