I don't think I would try to take an ICE (or IC/EC) connection to Saarburg. They are nicer trains, but they take a roundabout route and take just as long or longer.
There are a number of regional connection going to Saarburg via Mainz Hbf. From Mainz to Saarburg, the travel is entirely within the German states of Rheinland-Pfalz and Saarland, for which the Rheinland-Pfalz-Ticket, at 32€ for two people, will apply. An RMV ticket to Mainz, for two people, either on the S-Bahn or on a regional train, is 5,80€ each, or 43,60€ for the entire trip, a little less than the QdL-ticket, although it means purchasing tickets for two segments instead of the entire route. It doesn't matter if you don't change trains in Mainz, as long as the train goes through the Hbf there. The RMV ticket covers your travel to Mainz; the RLP ticket covers you the rest of the way.
If you have already reserved your hotel in Saarburg, and you don't mind getting in after 6 pm, there is a regional connection leaving the close-in Regionalbahnhof at the airport at 14:23, 1H23min after your scheduled arrival and getting into Saarburg at 6:08 pm. There is another connection, leaving at 15:24, 2½ hours after your scheduled arrival and getting into Saarburg 9 minutes before 7 pm.
If you want to avoid the evening arrival in Saarburg, another possibility would be to take the S-Bahn (5,80€/P) to Mainz and stay in one of the hotels right across the Bahnhofplatz from the Mainz Hbf. I've stayed here before and would recommend it. In the morning it's a short walk across the Platz to the station. You could use the 32€ Rheinland-Pfalz ticket from there. There is a regional train connection departing from Mainz Hbf at 9:51, changing in Koblenz, and getting into Saarburg at 9 minutes before 1 pm. If you wanted earlier, there is an 8:52 connection out of Mainz Hbf, changing in Saarbrücken, getting into Saarburg at 7 minutes past noon.
I think staying in Mainz would be preferrable to Frankfurt Hbf or the airport, because it is on the way and would avoid some backtracking and would save about an hour Saturday morning.
I don't know how comfortable you would be with going part way Friday evening and finishing the trip on Saturday. I've been through a lot of the towns on the Rhein gorge, and I would suggest St. Goar. I think it would be the most convenient. There are a lot of hotels just down the hill from the train station and on the river. It would take about an hour and a half to get there from the airport and about 3 hours to Saarburg in the morning.
Another suggestion, although I no have no hotels to recommend, would be Cochem. It would be just over 2 hours on your arrival day, so you could undoubtedly get there before dinner, and the trip to Saarburg the next morning would be just over an hour. I've been to Cochem, and as I remember, there are a lot of places to stay just south of the train station in town.
