So if I have it correct we can get the 7h28 IDTVG2967 or 7h28 TVG8531 departing same place going to same place but different trains or theTVG8533 at 1028 then get off at Bayonne & having brought travel onto Lourdes.
Well, at the moment & judging from SNCF website your choices are
As you can see only 6:28 train is the direct one, while all other options require one or two changes
How do you know what time the train gets into Bayonne to buy the leg to Lourdes and how much time would you need to transfer.
If you scroll down on the same page you would be able to see the exact schedule for all the connections. In this particular case Paris-Bayonne is 10:28-15:32, while Bayonne-Lourdes is 16:03-17:40, totally enough time to change trains. And no need to buy ticket for the second leg from Bayonne to Lourdes, since you can simply buy one through ticket Paris-Bayonne-Lourdes with the predefined train change
One last question is there any way to change your seats I not sure I got it right when I brought another ticket
As far as we know, you can't change seats on your booked tickets, especially when it's discounted tickets. However nobody forbids you simply take any free seat once you already on the train. If the train isn't full, then conductor won't mind that you've taken different seats from the ones listed in your ticket