Hello malct.
You seem very confused while the issue is very simple.
When you fly CX, or any other OW airline, you do not need to open an account with CX.
Just book as a guest on CX website and enter your QR, BA or other FFP number.
You can earn avios using either a QR, BA or AY FFP. Other airlines (like CX FFP) use a different currency and you cannot transfer those to avios.
As you mention, S fare class does not earn any avios on QR FFP but it does on BA FFP.
As you can easily transfer avios between these three programs, I would suggest that you create a BA FFP and later transfer those avios to your QR FFP.