buying train tickets online to trip to Rhine area

German Rail is running buses to add more trips or more direct service in some areas. These buses have a fine reputation. They are showing you the most efficient connections at any given time of day and you can choose whether you prefer faster travel time, fewer connections, or departing at a particular time of day. If train tickets are not sold online, then plan to buy them in a train station. Some of those legs from Koblenz to Oberwesel are operated by Mittelrheinbahn, a separate company.

German Rail is running some buses to add more trips or more direct service in some areas. They are showing you the most efficient connections at any given time of day and you can choose whether you prefer faster travel time, fewer connections, or departing at a particular time of day. These buses have a fine reputation. If train tickets are not sold online, then plan to buy them in a train station. Some of those legs from Koblenz to Oberwesel are operated by Mittelrheinbahn, a separate company.

"... tickets from Treis-Karden to Oberwesel the website shows the schedule but says 'fares not available'"

DB doesn't show fares for trips it doesn't sell. Treis-Karden to Oberwesel is a local trip within the VRM transit district. The trip takes 1-1.5 hours and costs €11.30 one way. You would buy this ticket from a machine at the station. There is absolutely no need for pre-purchasing - these tickets cannot sell out.

Is this a day trip? Maybe you want a day pass instead. It's good for all the trips you like in one day within the VRM transit area and it costs €21.80 for a "mini-group ticket" or "mini-gruppenkarte" (again, a day pass really) and you buy it from a ticket machine.

Are you spending several days in Treis-Karden, or in the area? Then maybe you need a 3-day mini-gruppekarte. That costs just €43.60 (you get a free day in other words.)

Read about the mini-gruppenkarte and the 3-day version HERE.

THIS MAP shows you the area that's covered by the VRM day passes.

"If we search for trip Offenburg (near Strasbourg) to Strasbourg it doesn't use bus, it uses Südwestdeutsche Verkehrs-AG Direction."

DB shows you the most efficient route. For T-K to Strasbourg, that includes a bus trip. If you want a train only route via Strasbourg, simply enter Offenburg as a stopover with no stopover time and you'll get a trains-only itinerary.

To find saver fares on this trains-only route, buy a ticket to Kehl (which you can sometimes get for €49 for two.) This will provide you a short stopover in Offenburg. You then need tickets from kehl to Strasbourg. A 24h Europass family mini ticket (which you can purchase at Offenburg station or in advance from DB, more conveniently, for €10.90) will cover the trip from Kehl to Strasbourg. As long as you take a regional train from Offenburg to Strasbourg, this 24h ticket works fine and there's no need to get out in Kehl - just stay on the train from Offenburg to Strasbourg.

24h Europass family mini ticket

Thanks for the helpful advice.

VRM (Verkehrsverbund Rhein-Mosel) is a large transit district covering the Rhein from Oberwesel to the Rheinland-Pfalz border with Nordrhein-Westfalen between Remagen and Bonn and the Mosel up (south) past Cochem. You can get schedules and prices from their English website. Note: the border of the VRM is between Oberwesel and Bacharach, so Bacharach is outside the VRM. If you had put in T-K to Bacharach you would not have seen "fares not available".

I've spent alot of time researching the best way on the Bahn site to get from St. Goar area down to Colmar in France. There are multiple possible routes and ways to get to that destination. Here is one possible journey we might go with:
St. Goar to Mainz Hbf via Mitterrheinbahn - switch platform (track)
Mainz Hbf to Freiburg Hbf via Eurocity - switch platform
Freiburg to Breisach via Breisach-S-Bahn
then bus to Colmar via DB bus 1076
The fare that the Bahn site came up with is 49 euros for two people. It says 'partial fare' and it looks like the bus fare is not included. The duration of the trip would be about 5 hours and involves the 3 changes.

We're new to traveling on trains and buses in Europe. Does the above itinerary look like a reasonable and efficient way to get from St. Goar to Colmar?

That sounds like the normal route to me.

You would use the Euregiobus between Breisach and Colmar. €5.30 each one way, €8.50 round trip. See flyer in French and German:

Flyer for Colmar trips

Colmar does take some time from St. Goar. I don't know enough about your whole trip to advise you any further other than to say there are a several nice old-world towns you might see that aren't as far or as touristy, including Gengenbach, near Offenburg; you could reach G'bach in under 4 hours from St. Goar:

Gengenbach, "main drag"
Gengenbach, Engelgasse

You can probably find a ticket to Gengenbach for two for €69 total.

Schiltach is a nice outing from Gengenbach. Outings by train or bus from Gengenbach to Schiltach, Freiburg, and other Black Forest towns are free if you stay in Gengenbach or other small villages that participate in the Konus program:
KONUS info.

I see some connections by MRB directly (no change of trains), but you say you would have a change. Looking at the schedule, I see only two change stations between St. Goar and Mainz - at Oberwesel or Bingen Hbf.

Oberwesel is a pretty simple change. The station has only two tracks, one in each direction. You get off the MRB in Oberwesel, stay on that platform, then get on the RE to Mainz on the same track a few minutes later.

Bingen Hbf is not as simple. First, make sure you get off at the first Bingen station, the Hauptbahnhof (Hbf). The next station in Bingen is the Stadtbahnhof. It's closer to the K-D dock, but the RE to Mainz is a limited stop train and does not stop at the smaller Stadtbahnhof.

The Hauptbahnhof has a unique track plan. There are six tracks with the station on a wide platform with three tracks on each side. The tracks are labeled 101, 102, and 103 and 201, 202, and 203, with 101 and 201 adjacent to the station platform. 102 and 103 are on their own platform adjacent to 101; 202 and 203 are on their own platform adjacent to 201. It looks like your train will come in on 201, to the right of the station coming from St. Goar. You get off the train and walk across the platform to track 101.

Thanks for the information on Gengenbach. That sounds good but we're committed now to going to Colmar.

At this time my concerns are dealing with the needed changes of trains and tracks at the various stations and how to get the bus ticket to Colmar from the Breisach station.

I bought my ticket to Colmar from the bus driver at the Breisach stop.

"my concerns are dealing with the needed changes of trains and tracks at the various stations"

At the DB itinerary page... did you click on the white-on-red arrow at the left of your itinerary? The platform scheduled numbers are provided there. German stations are well-signed so look around and you'll see something. On the platforms, look for signs like THIS one. The big "1" at the left means platform 1. ("Nord" means the north end of the platform. Final destination is München - Munich.) In THIS PHOTO the platform numbers are given along with a letter ("F" in this case) to indicate the relative location on each platform (getting on the correct car for your reservation might require you to stand at location A, B, C... or F.) There may be other directional signs too. "Gleis" or "Bahnsteig" are the words commonly used for platform. The escalator under THIS SIGN gets you to platforms 7, 9-10.)

Occasionally there will be a change in platform number from what was scheduled. Larger stations have an electronic board like THIS ONE somewhere in the main hall that lists upcoming departures and current platform numbers.

I'm not even going to offer a detailed explanation of changing trains. It's so intuitive and easy that you'll get it immediately. Don't overthink this.