Suggestions for Rhine/ Mosel Trip

Very doable. On the Rhein, the trains run on both sides of the river and stop in virtually every town. The boat is a lot slower, particularly in the direction of Koblenz to Bingen, but not too bad in the other direction. I think the K-D boat starts at Mainz, not Frankfurt. Some people like to start in Bingen and go down to Koblenz (~3½ hrs) but I would prefer to start in Bacharach (2½ hr to Koblenz), or just take the boat 40 min to St. Goar and the train the rest of the way). K-D has a boat to Cochem but I think there is another line that goes further up the Mosel. Most of the towns commonly visited above Cochem (Beilstein, Traben-Trarbach, Zell, and Bernkastel-Kues) are on the river. Because the Mosel loops around so much, and the train only goes along one side, seeing those towns might be easier from the river. You can also get to them by train and bus from Cochem. On two trips, I have spend 7+ days (9 nights) on the Rhein and Mosel, although I did not go up the Mosel as far as Trier. One misconception about the Rhein boats is that they are like hop on/hop/off buses. There are only three boats a day from Bingen all the way to Koblenz, so you could only get off in 2 places. If you want hop on/hop off, you're best with the hourly trains. One of the most interesting places to visit, in my opinion, is the Marksburg castle in Braubach. It's on the right bank. If you not taking the boat all the way down, you should take the ferry across from St. Goar/Rheinfels to St. Goarshausen (on the right bank) and the train from there to Braubach.

Yes, Use the local trains (regional bahn) to get around. The boats are slow but work great for siteseeing between St. Goar and Bacharach. There are two types of cheap train passes that work great for your trip. The Laender (translates to state or province) Pass is good for one province and allows up to five people to ride cheaply (after rush hour through midnight) with unlimited stops. The price and rules change slightly by region (it should be under 30 euro for a full day of transportation for your group). The Schoenes Wochende (Happy Weekend) Pass is similar but is only for travel on Saturday or Sunday and isn't limited to one province.

Along the Rhine the trains are very useful and quite secnic. However you must take a short (at least) boat trip.... just cause you are there. And just cause i am a tourist, I would make sure it is along the Lorelei stretch.