Heidelberg to X to Frankfurt

what month will this be?

In the month of May 2019

You will need to go to Bingen (Rhein) Stadt. Bingen has 2 stations, Stadt and Hauptbahnhof. Stadt is closer to the K-D boat dock. Assuming the trip is April 13 to October 20, the K-D Rhine cruise boats will be operating, then yes, it is a good plan.

There is an hourly regional connection from Heidelberg to Bingen Stadt at 23 past the hour, with connections in Mannheim and Worms costing 25.10 EUR per person. Show your train ticket and get a discount on the boat ride.

St Goar to Frankfurt is by frequent trains. With a regional train, connecting in either Bingen Hauptbahnhof or Mainz Hauptbahnhof, trains are once per hour costing 22.80 EUR per person.

There are other trains that do these routes with IC or ICE trains. If you buy advance nonrefundable tickets, they can be cheaper, but cost more bought day of travel. For these short distances, they really don't save any time. Schedules and prices at:
https://www.bahn.com/en/view/index.shtml
Click on "further options" then "local transport only" to see the regional train schedules.
KD boat schedule is at http://www.kdrhine.com/rhineschedule.htm

Thank you, Sam, for your insightful answer. I have a Germany rail pass, so I simply need to show that, right?
Also, any insight on questions 3 and 5. Thank you in advance.

Re 3. I think you have it, take the boat from Bingen to St Goar.
Re 5. Visit one of those castles, like Burg Rheinfels by St Goar.

You do get a 20% discount on the KD boats with a German Rail Pass.

You cruise the Rhine on day 1 . Why are you doing it again on day 2???

Thank you Sam for the answer.

Hi Stephen, on day 1 I'm just taking the ferry from Bingen to St.Goar. Once in St. Goar I plan to explore nearby places/castles/etc (need ideas)
On day 2, we take the cruise that will show us the castles etc. (need ideas here too on what else we can do)
Do you have any better plan in mind? Please share.

"day 1 - Heidelberg to St. Goar (Heidelberg to Bingen & then K-D line ferry to St Goar, stay at hotel) (Explore Bacharach, Oberwesel, walk to Rheinfels castle)"

You have planned too much for this day.

It will not work well for you to tour Rheinfels and to explore both Oberwesel and Bacharach on this day. The train ride to Bingen via Mannheim and Worms which Sam suggests is 2 + hours. I'm finding departures on my choice of dates at :42 or :43 after the hour. Let's say that after breakfast you left H'berg on the 8:43 train, which puts you in Bingen Rhein Stadt at 10:56, and which means catching the 11:30 K-D line boat from Bingen. And let's say you take the boat straight to St. Goar without stops (arrive 12:55), check in at your hotel, find lunch, then walk up to and tour Rheinfels Castle. By this time it's at least 16:00 - even without having set foot in Bacharach and Oberwesel, much less "explored" them. And if, on the way to St. Goar, you DID choose to leave the cruise boat to explore them, you have two problems... 1) the luggage that you are carrying - ugh - and 2) the skimpy boat schedule. Getting off in Bacharach means a required 3-hour stopover there, and in Oberwesel you'd have a 2-hour stopover, with final arrival in St. Goar at 17:55 - and - whoops - while you were toting your bags around these towns over 5 hours and cruising, Rheinfels Castle closed its doors.

"day 2 - Explore + Rhine valley cruise"

You are confused, I think... The "Rhine Valley Cruise" and the "K-D-line ferry", as you call it, are the same thing. KD operates cruise boats, not ferries. You don't need to cruise today if you cruise Bingen - St. Goar, the best part of the river, on Day 1.

So I would re-organize your trip a bit...

Day 1: train to Bingen, cruise to St. Goar, check in, visit Rheinfels Castle. If by late afternoon you still wish to spend some late time in Bacharach, it's only 10 minutes away by train. (You can get a train back to St. Goar until about 23:30 or so.)

Day 2: Explore by train... You could visit Oberwesel, Boppard, and/or Braubach (Marksburg Castle!) very easily from your St. Goar base. Oberwesel is 5 minutes by train from St. Goar, Boppard is 15 minutes to the north. If you visit Braubach, use the real ferry to cross the Rhine from St. Goar over to the east bank train station in St. Goarshausen + a 20-minute train ride to Braubach. The ferry runs all day, and trains to Braubach are hourly or better.

Boppard
Boppard chair lift
Gedeon's corner (a short walk from the chairlift)

Braubach
Braubach town map (DB = station)
Marksburg, an intact medieval castle

Walk around Oberwesel

What if you don't have a spare railpass day for these train trips?? You can get a cheap all-day VRM day pass that covers the St. Goar ferry and all your trains from a ticket machine at the St. Goar train station:

http://www.vrminfo.de/en/tickets/tickets/ticket-offers/leisure-ticket/

Thank you so so much, Russ for the wonderfully insightful and practical comment. You're totally right, I didn't plan the days right. And your comment made it crystal clear. I'm going to follow your itinerary for those two days. Cheers, mate!

"I'm going to follow your itinerary for those two days."

OK, but check DB schedules thoughtfully for your exact travel date.

And here is the St. Goar FERRY schedule:
http://www.faehre-loreley.de/fs_fahrplan.html
Scroll down for timetable... "ab St. Goar" = departures from St. Goar