Is this a workable Rhein Tour Day?

2) Drive/train to Braubach; see Marksburg castle (10 minute ride; 30 minute walk up hill; 50 minute tour; 20 minute walk down) when opens at 10:00.

Ten minutes from your hotel in Koblenz sounds far too optimistic. It will take you a little bit of time to get out of the city, and I'm pretty sure Braubach is a little further away than 10 minutes.

Also, if you have a car... there's a parking lot right below the castle near the top of the hill. If you don't want to walk all the way up, there's no need.

Finally... English tours aren't very frequent. Unless you can settle for German, you need to time your visit carefully.

I would train farther down the river to take the ship back north. You are missing all of the castles if you only train to Ober-Wesel or Bacharach, both of which are only about a 5-10 train ride from St. Goar.

Train from your hotel or Koblenz to Braubach, do the English tour. Continue by train down the right side of the Rhine to Rüdesheim or Assmanshausen. Take the ship down the river then to Bacharach. Have lunch there, then continue by train to St. Goar. There aren't a lot of nice places to eat in St. Goar and the town seems to have lost a lot of their stores and businesses. I haven't found it to be very attractive the last few times I was there. The last time was in May, but there is a lot of construction going on. Bacharach and Ober-Wesel are prettier and have more places to dine. Last entry to Burg Rhinefels, which is really worth visiting, is around 16:00 I think.
Finish by training back to your hotel near Koblenz.

There are dozens of way to slice this pie. You're going to see some nice stuff. What I don't quite understand is why you have decided to stay in Koblenz, a relatively dull place, overall. The smaller Rhine villages and the hotels there have much more character than Koblenz, which looks like it was rebuilt very quickly after WW II.

Is Koblenz really where you "need to be on the morning of Day 2 (beginning of a trip up Mosel River")?? As you know, Braubach is only 10 minutes from Koblenz - that doesn't seem burdensome. But the main thing is that Braubach is really a world apart, a lovely old town in a scenic spot, so why not just drive straight to a hotel or inn there instead of messing around with the time-consuming logistics of much larger Koblenz? In Braubach, everything is right there - castle, train station, boat dock. Also, at the end of your cruise day there's no real merit in cruising from Braubach to Koblenz for another 50 minutes - it's not very scenic, and often industrial.

I completely agree with Jo about starting your cruise further south. Well, almost completely. I'd start the cruise later in the afternoon, not in Assmannshausen, but either in Rüdesheim, or Bingen, so that you don't miss the outstanding scenery immediately north of there just out of Bingen.

Before that it would be smart to see St. Goar and/or Bacharach and/or Oberwesel by train - so do what you suggested... After Marksburg take the train south to St. Goarshausen (20 min.), ferry across to St. Goar for Rheinfels (but don't do Rheinfels if the DW is going to get tuckered - it's a fairly steep hike up again.) Then see O'wesel and/or Bacharach by train and then catch a train to Bingen before that last boat leaves at 16:30. I'd probably ride it all the way back to Braubach (17:20.)

Let me just defend Koblenz for a second. There are some very ugly modern commercial streets, but there's also a wonderful old pedestrian-only shopping area, some beautiful gardens, a very pleasant river promenade, and some rather attractive German Empire period residential neighborhoods. Plus, being a city, more nightlife and dining options than any of the small towns to the south. I can think of far worse places to spend the night than Koblenz. It's not a small town, but if you'd rather base your Mittelrhein experience out of a city, it's a very satisfying choice.

There is a little tourist tram that can take you up to Rheinfels - saves a lot of energy !

Mffoley, I wonder if you are picking up a sleep-aboard Mosel cruise, or just a day trip. I urge you to consider whether the car is of any real use to you.

We happen to have slept in Andernach when we had a flower show in Koblenz, and our touring was less vigorous and less pre-planned than yours. But you can assume that the train will be faster and cheaper than the car. The only problem if you don't have a chip credit card is the ticket machines. But you can use cash, if you have the right denominations. The Koblenz stations should have live agents.

Anyway, we parked in Andernach and didn't touch the car until we moved to a Mosel hotel, via Burg Eltz, three nights later. In that more rural town, the car was useful and easy to drive. I wouldn't want to drive in Koblenz.

You should download the KD boat schedule to see why you want to travel downstream. As noted, any boating outside Bingen->Boppard is a waste of time. And because you prefer to schedule your day carefully, time is valuable.

I wasn't exactly attacking Koblenz... it's just that except for its location on the Rhine (and Mosel) it's an extremely average place that can be problematic for visitors using boats and trains. Most of the "wonderful old pedestrian-only shopping area" is not "old" - it looks like THIS. There are a couple of nicer blocks of old Koblenz left, closer to the Mosel river, but that part and the shopping part as well are a healthy distance from the main train station (which the OP plans to use) and the station is an even healthier distance from the KD dock. Braubach on the other hand is very compact and simply more charming overall - with just one night in the area, it's a better choice. In WINTER when the smaller towns are deadsville and the KD boats have been stilled, Koblenz makes good sense as a base town for the area. But it's still summer for the OP, who is planning a full day on the river and wasn't going to arrive at the KD dock until after 20:00... and I suspect from his comments that nightlife is not in the cards.

Most of the "wonderful old pedestrian-only shopping area" is not "old" - it looks like THIS.

That's not what I referenced, but a picture of the relatively bland modern shopping street. This is what I meant. And this. And this. And this. And this. And this. And this.

"And this. And this..."
And those photos represent the relatively small, relatively nice part I referred to as well, the part near the Mosel (and the part that the OP won't have time for anyway after their long day on the Rhine.) HERE is a map of Koblenz. The pedestrian zone streets are in blue. The "Löhrstrasse" stretching from the Herz-Jesu-Kirche is the modern shopping zone. As you approach the Liebfrauenkirche, you finally arrive at the area where Tom's photos are take. It would be an attractive area to stay in and for hanging out, with some nice restaurants, cafes and bars - if one had the time for that, that is, and if one didn't mind going back and forth on foot or by bus through the rest of Koblenz to the main station and the KD dock (and if one had an extra 50 minutes to spend on the cruise boat and didn't mind paying extra for the mostly disappointing views between Braubach and Koblenz.) Pleasant as Koblenz' small old town zone is, I really think mffoley will find it easier to achieve his clearly-stated goals for the day if he overnights in Braubach.