It will cost you over $500 US to fly one-way from CDG to Pau on Air France. It would be cheaper to book a roundtrip ticket and throw away the return portion.
If you were to take a TGV from CDG to Lourdes rather than going into Paris and departing from Gare Montparnasse, you wouldn't be able to leave until 13:37, you would have to connect to another train at Bordeaux, and you wouldn't arrive at Lourdes until 21:16.
I don't know how many other train rides you plan to take in France, but if you can commit in advance to specific departure dates and times, you can get discount fares for many routes, thereby making a railpass not cost effective.
If you take the 14:40 TGV directly from Gare Montparnasse to Lourdes, you'll arrive at 20:14. The standard 2nd class fare is €91.50. If you book your ticket well in advance (up to 3 months allowed) at www.tgv-europe.com, you can get a 2nd class Prem's fare as low as €22 and a 1st class Prem's fare as low as €40. To keep the site in English and to avoid being bumped to the Rail Europe site which doesn't offer discount fares, choose Great Britain as both your country of residence and the country in which you will retrieve your ticket. If you get a Prem's fare, you print the ticket yourself and take it with you. Since you don't want your ticket mailed to the UK, for any other fare choose to pick up the ticket at a station automatic ticket machine. Note, however, that those machines only take credit cards with embedded chips so you'll have to go to the window and have the agent swipe the card you used to book the ticket and you'll need to give the confirmation number you'll be sent by e-mail.