We are planning a trip, by train, to France & Spain. Paris, to Bayeux, to Mont Saint Michel, to Amboise, to San Sebastian, to Pamplona, to Barcelona. Is it best to purchase all tickets before we leave? Is a rail pass a good idea?
You don't need a rail pass. It is best to buy in advance whatever long distances you need to travel on high speed trains because those trains require reservations and the more people reserve the higher the prices go up.
The Amboise-to-San Sebastian leg will be a 9+ hour slog by rail, and I don't think there's a good air solution. You might explore stopping for a couple of days somewhere along the way to break up that trip. Maybe Bordeaux?