It might be useful to ask your hotel in Tours if they happen to know a taxi or private driver who could take you around while you are there. We have done that before with success in other regions of France and have been able to negotiate flexible and cost effective trips that way.

So best to buy discounted fare since nothing will be refunded even with full-fare ticket? If miss that train you'd have to buy another one at full fare. Best IMO to stay 1st day in Paris then lesiurely get train from Gare Montparnasse.

You probably should considering taking a taxi or RER to Montparnasse station in Paris and then buying tickets on any of hourly trains - unlikely to be sold out especially in first class. Trains from CDG toSPdC are sporadic but if the next train is only a few hours later maybe buy full-fare on that train and then change it if you arrive for earlier train.