"We arrive in Frankfurt at 8:40 Sunday morning, and then were going to head straight to the Rhine and hop on hop of the train to stop on our way to Koblenz for the night... We are looking to stop in Bacharach, St. Goar and Boppard."
There might be a bugaboo or two in your plans.
Presumably, you have luggage, and your plan is to lug it around these 3 towns while you sightsee. None of these small towns have station lockers. TI offices may or may not be open to hold your bags for you, but you still would have to mess with getting the bags to 3 different TI offices. So I'd strongly suggest that you book a room in Bacharach instead. Drop bags at your hotel. THEN visit St. Goar (10 minutes to the north) and Boppard without the hassle. The alternative, dropping bags in Koblenz first, will mean a lot more time on the train. Note also that Koblenz is just an average post-war city without a whole lot to commend it. Bacharach (or one of the other small Rhine villages) would offer a much better atmosphere.
"Then an early train and hike to see Burg Eltz right when they open to try and get back to meet the KD boat."
KD boat from where to where???? If you're catching a train back to Koblenz for a Rhine Cruise, this is problematic.
- The boat dock is a long walk from the station in Koblenz.
- If your plan is to catch the 14:00 boat in Koblenz, and you want to see the best part of the river (between Boppard or St. Goar and Bingen,) you will be on the boat for SIX HOURS. The first 2 hours south of Koblenz are fairly dull and industrial in parts. And you will pay a lot more for your cruise than the smart cruiser who targets just the best part.
- The smart cruiser takes the train south to Bingen, boards there, and ends the cruise in St. Goar (1.5 hours) or Boppard (2.3 hours.)
It sounds to me like you should re-organize this thing altogether. Here's one workable plan:
From FRA you take a train to Bacharach, Oberwesel, or St. Goar for overnight. Drop bags. Settle in, get some food. Catch the train south to Bingen and board the 14:30 cruise north to St. Goar (13:55.) Sightsee in St. Goar, Oberwesel and Bacharach that day - they're all just a few train minutes from each other.
The next day, do Burg Eltz and stop in Boppard on the way back for sightseeing.
If you CANNOT change your Koblenz room... Then on Day 1 take the train to Bingen. Board the next cruise boat to St. Goar. Board the train there for Koblenz to drop bags. Take the train south to Boppard for sightseeing if you're still up for something. Day 2: Burg Eltz, then St. Goar and Bacharach sightseeing by train. (Not the best 2-day plan but possible if you're stuck in Koblenz.)