One day tour on Rhein from Frankfurt

Plan B: I take it this is a weekday. On a weekend you could just do the weekend ticket €44/all trains for two + the KD boat (€20 price should be €16 each at the dock with the 20% discount you get by showing your train ticket.)

Plan B: You have under-priced some items I think.

RMV Gruppentageskarte for Frankfurt to Ruedesheim is €34.50. You have €26??
The KD fare is €20, with a 20% discount €16 each.

I would probably do this instead...

Have breakfast in Frankfurt. Leave Frankfurt 8:47, change trains in Mainz, arrive BINGEN Rhein Stadt 9:59; walk to KD dock to board 10:30 boat (the same boat as your 10:15 in R'heim.) Proceed as previously outlined. Stop over in Rüdesheim on the return trip if you wish.

Buy RMV Gruppentageskarte for Frankfurt to MAINZ - €28.30 - which later will be valid for Wiesbaden-Frankfurt as well. Also buy Rheinland-Pfalz ticket for two - €29 - valid for Mainz-Bingen, St. Goar ferry, train to Braubach, and train to Wiesbaden (via Lorchhausen, Rüdesheim.) €57.30 + €32 KD (pay at dockside kiosk) = €89.30

Whether you think the Cologne cathedral is worth the time and expense is really a personal preference, I think.

On your option B just a couple of nits.
St Goar to Braubach via the ferry to St Goarshausen is a VRM Preistuffe-8 ticket and is 9.65 EUR per person.
Return to Frankfurt, you can stay a bit later in Braubach and take the 18:06 train all the way to Frankfurt. It is the same train you are planning to catch in Lorchhausen. You just have to pay for the ticket from Braubach to Lorchhausen and stay on the train as your RMV tageskarte will kick in. Now you will have spent 9.65 X 2 plus 8.90 X 2, which is 37.10, so you might as well buy a Rheinland-Pfalz ticket for 29 EUR instead.
And I am getting a price of 34.50 EUR for an RMV Gruppentageshkarte for Frankfurt Hbf to as far as Lorchhausen and Ruedesheim.

What's the first place that I can get off the DB train and get the RMV to Frankfurt?

This is probably a moot question, since one of several Bahn Tagestickets (SWT or QdLT) will probably cost less than an RMV day ticket and Bahn tickets, but any Bahn regional (local) train running in a area controlled by the RMV is under fare control by the RMV, so all you need is the RMV ticket. No need to change trains. Coming back along the Rhein, the first station under RMV fare control is either Lorchhausen or Bacharach, depending on which side of the river. I say "RMV fare control" because those towns are actually in another transit district, the RNN, but in an area where both RNN and RMV tickets are valid.

I think your second itinerary works better for seeing the Marksburg. You can only tour the Marksburg with a tour and the English tours are at 1 and 4 PM (13:00 and 16:00). Of course, you could always do the tour in another language, but the second itinerary gives you an hour to get up to the castle (enough time in my experience) and book the 16:00 tour, then enough time after the tour to get back down to the station. I'm also not sure that 2 hrs is enough time to go up and back and take the tour.

Thank you all for the replies. They were very helpful.

I was having a hard time finding information in RMV site. I didn't know about Rheinland-Pfalz ticket and that it covers the St Goar ferry. It's definitely the best option.

Although Cologne is a very walkable city from the HBf, 3 hours there is a desperate rush. Will you ever visit this part of Germany again in your life? You can see the Cathedral interior, but you'll only have time for a hasty glance inside ONE other of the many museums, or the optional (pay) Cathedral treasury if you prefer that. I would go to the Roman museum, myself.

I'll try to give a last shot at nickel-shaving. This must be a week day or a Schoenes Wochenende Ticket would be the way to go.
Buy 2 RMV tickets for 4.80 EUR each to Kelsterbach, and 2 RMV tickets for 8.35 EUR each to cover Wiesbaden Hbf to Frankfurt. That is 26.30 EUR in individual RMV tickets, and less than the 28.30 EUR for a Gruppentageskarte. Now buy a Rheinland Pfalz ticket for 2 for 29 EUR. Total train ticket outlay 55.30 EUR.

Now take the 8:47 S-8 to Maniz Hbf. The train will arrive at Kelsterbach at 9:02, so the rest of the way to Mainz is covered by the Rheinland Pfalz ticket. At Mainz Hbf, change to the 9:32 Mittelrheinbahn to Bingen Stadtbahnhof arriving at 9:59. That connection time at Mainz Hbf is 7 minutes from Track 1a to Track 11, so hustle!

It is a 10 minute walk to the KD dock (according to DB) and the boat will be there at 10:30. It is the same boat you were planning to catch in Ruedesheim at 10:15, but it just crossed the river and Bingen is the next stop. Proceed with the day as planned.

Returning from Braubach, you can get the 18:06 leaving Braubach and arriving at Frankfurt at 20:05. The Rheinland Pfalz ticket is good to Wiesbaden Hbf, then the RMV tickets take over for the rest of the way to Frankfurt Hbf.

If, as Lee suggests, you need more time at Marksburg, there is another direct train at 19:06.

Great advice by Sam. But ...

Buy 2 RMV tickets for 4.80 EUR each to Keltersbach

... please tell the ticket machine that you want to go to Kelsterbach

Spelling mistakes are my bane. But I will edit it.