Pippa,
The only reason you would need to use the CSA, is if there was disagreement about how much money was needed to support your stepdaughter - and then only if one parent was not willing to pay anything. The role of the CSA is to chase errant parents who do not want to pay support for their child. They are a really badly organised institution and I would avoid them like the plague if you are able. Believe me, they are one step down from useless. Rant over!!
You need to see a family lawyer who will be able to iron out the finer points that you have mentioned.
I have a feeling that the child benefit will still be paid only in one sum and to the same person every month, so you will have to come to an arrangement reagrding this.
Good luck to you and I hope that the two weeks on/off works well for all concerned!