Question 1: Yes, you have plenty of time if your flight arrives on time.

Questions 2 & 3: It depends on the TGV ticket that you purchase. A full-fare TGV ticket allows you unlimited changes (assuming, of course, that seats are available). Some discounted TGV fares also allow 1 change per ticket; other discounted TGV fares do not permit any changes. Note also that if you buy a TGV ticket online from voyages-sncf and you choose to print your ticket, your ticket becomes non-changeable, even if it would otherwise be an unrestricted fare.