No, there is no SBB travel center in Paris.
No, you don’t have to validate your Swiss Half Fare Card when purchased as online ticket. It is active from the chosen validity start date. Kindly note, it is a card not a pass, it does not grant free access to the Swiss public transport network.
If you need the complimentary Swiss Family Card, you can get it at the first station in Switzerland when arriving from Paris, being Basel SBB, Geneva, or Lausanne.