When you had the tank replaced did you have the system bled? Did you change the expansion tank cap? As seals can fail and you wouldn't know if it was leaking from there. Could there be an issue with the heater matrix? (Any wet carpets etc?)
S line is just the spec. Rs4 is a wider car and different bumper support etc. If you want an rs4 style bumper then I'd recommend you look for an aftermarket one. Obviously this would be classed as a modification and insurance would have to be notified
That's the thing with Audi, Yiu get less spec for your money compared to VW. Yep they can be retrofitted I'd recommend speaking to autologics or Audi retrofits