You can add and remove trains from your pass as much as you please at any point, just as long as you have it added to the train you are actually using before you board.

With trains that do not require reservations, such as those above from Strasbourg and Basel this is all you need to do.

If you miss one train just check the departures board and get the next one.

It isn’t needed for this connection as those trains are reservation free but if you are delayed and miss a reserved train then the best option is to speak to staff at the station and they should get you transferred to the next service, you shouldn’t pay for a second reservation if you have been delayed by the railways.

There is no Passport control, you are travelling within the European Schengen area, there can be spot checks but there are no formal border posts or airport-style checks to slowly go through and no need to be at train stations hours before departure.

Indeed. If you arrive about 20 minutes in advance in Paris-Est it’s perfect for the TGV. You can proceed easily to the correct train, without the need to wait. The SNCF only displays the platform about 20 minutes before departure. The only check that happens before boarding in these stations, is a short ticket check.

This community is awesome. Thank you both for your quick replies and detailed explanation :)