Seat reservations in Germany are only possible on long-distance trains (ICE, IC, & EC). I don't see much of your routes utilizing many long-distance train sections.
For the first day (FRA to Strasbourg) you could use an ICE, either all the way (change in Mannheim) or to Offenburg, but I would not advise using a Saving Fare ticket (requires at least one leg on a long-distance train) because those tickets are train specific and nonrefundable if the flight is late. However, if you use a participating airline (Lufthansa is one), you can book a rail&fly ticket for about 33€ per person, which will give you an open ticket (no specific train) on the day of arrival to any station in the Bahn system.
For Strasbourg to Schwalmstadt, I see plenty of Savings Fare tickets for 58,00€ using a TGV to Frankfurt and regional trains from there. Since you will have been in Strasbourg for several days, you should have no problem making a specific TGV.
Schwalmstadt to Bacharach is best by regional trains (no reservations) using a Quer-durchs-Land-Ticket (52€ for two, weekdays) or a Schönes-Wochenende-Ticket (50€ for two, weekend day).
Bacharach to Cochem is by regional trains using a Rheinland-Pfalz-Ticket (29€ for two).
Cochem to FRA is also best by regional train (2 REs with change in Koblenz). Rheinland-Pfalz-Ticket to Mainz, then RMV local tickets to FRA at 4,90€ per person.
What time is your flight out of FRA on the last day? Do you really plan to come all the way from Cochem to FRA that day?