The France 5 day flex-pass meets the needs of my wife and I, and would be less expensive than point-to-point purchases, yet the difference in cost ($234) might be more than made up with the additional cost of seat reservations where required and additional hassle and/or ability to obtain them. I read commentary where seat reservations are limited for pass holders in France and can be hard to obtain, perhaps requiring one to take a train with a much less desirable schedule. My wife and I know our desired schedule and will be traveling in September on TGV, Express Regional and Inter-City trains and could purchase rail passes and make necessary seat reservations now. Or, I can make point-to-point purchases, with included seat reservations, now. My questions are 1) Would seat reservations on TGV trains (will be on no less than 3) be difficult to obtain now and what would the approximate cost of each seat reservation be?, 2) Are seat reservations required, or a good idea to obtain if possible on Express Regional and Inter-City trains, and what would the approximate cost, if any, be?, and 3) Finally, with the information I have provided, from recent experience, for us, would one recommend the pass or point-to-point purchases?
I see you've been on the forum for some time, but I just want to be sure you have priced out the point-to-point tickets on the SNCF website or with one of the sellers that charge the same fares (trainline.eu or loco2), rather than RailEurope. I'm surprised at the fare gap you have found for travel in September.
I believe you do need reservations for InterCity trains. You do not need and cannot make reservations for TERs.
The fees are a lot higher for first-class seats.
With respect to your first question, seat reservation fees for TGVs range from US$12 to US$109 per person or so says RailEurope on its website. On other French trains, the fees range from US$2 to US$12. If you are taking any international trains like Eurostar or going Lyria, those reservation fees range from US$11 to US$40. You can find reservation fees on the RaiEurope site at https://www.raileurope.com/book-now/article/passholders-seat-reservation-guide. You will have to check each trip and see what the reservation fees are to see if they will amount to more than the savings on the pass. I can't really answer your question without knowing where you are traveling between because of the range of the fees. Reservations are usually available 60 to 90 days before travel dates. Whether any given reservation will be difficult to obtain really depends on when you are traveling and how many pass holders are traveling that day and how many of them book their reservation before you do, but my guess is that if you book on the first available day and are flexible about the time of the train that you take, you should have no trouble getting a seat reservation.
With respect to your second question, for InterCités trains, reservations are mandatory or recommended (depending on routes).
Trains with mandatory reservation: Seat reservation fee = US$12
Trains with recommended reservation: Seat reservation fee = US$2
Higher reservation rates and artificially restricted access are only on international trains, which your pass would not cover. Within France, domestic TGV reservations never go higher than about $27, as trains get fuller, but you're more likely to pay $12-14 with some advance notice. They're much more readily available than they were a couple of years ago. For instance, several departures on the popular central TGV line are reservable in either class tomorrow morning from Paris to Perpignan. You can add some seat reservations at the same time that you buy the pass, before checking out, and get others in train stations as you go.