mgmbiz: Nice explanation of how you used the VRM Guest Ticket for outings. I would only add that the fee/tax that you mention is collected whether you use your free ticket or not.
One of the days we took the train from Boppard to St Goar, toured
Rheinfels, then St Goar to Oberwesel, did the walk along the medieval
town walls, then Oberwesel to Bacharach for lunch.
St. Goar and Oberwesel lie within the VRM zone and are legitmate destinations for the Guest Ticket. Bacharach, however is a few miles outside the VRM zone - so if you went there with only your Guest Ticket, you were actually traveling between Oberwesel and Bacharach as a "Schwarzfahrer" (anyone traveling without a valid ticket) and if caught, you'd have paid a hefty fine. To make things right, you would have needed a standard one-way ticket for Oberwesel > Bacharach (only a few Euros,) which you could have purchased at Boppard station prior to boarding.
Here's a map of the VRM zone destinations. All are inside the light-green VRM area.
In Bacharach we boarded a KD river boat back up to Boppard. The boat
was more expensive than we thought it would be, but worth the
experience to be viewing the beautiful castles from the water.
It is possible to cruise any specific segment you wish to cruise with KD - and prices vary by distance. But given a choice, I'd recommend to others reading this post that you start the cruise further south in Bingen. It's fairly common wisdom among those forum members who are familiar with the Rhine cruises that Bingen > St. Goar is the most intensely scenic part. If you only cruise north from Bacharach, you will miss HALF of this most scenic part. Here's a photo of some castle/vineyard scenery just north of Bingen that mgmbiz skipped:
https://upload.wikimedia.org/wikipedia/commons/e/e3/Maeuseturm_Burg_Ehrenfels_Bingen_Rhein.jpg
Bacharach > Boppard is a "G" fare (€20,80.) For the same fare you can do the top-drawer part, Bingen > St. Goar. If you are staying in Boppard, you might want to pay a little more to cruise all the way to Boppard after that... Bingen > Boppard is an "H" fare (€29.80.) That said, the Guest Card allows free train travel from St. Goar to Boppard - so you might just switch to the train after a Bingen > St. Goar cruise instead and pocket the extra €9.
(One other way to save on the cruise... take the TRAIN to Bingen or other starting point and show your train ticket at the KD kiosk for a 20% discount on the fares I named above. Not sure whether mgmbiz did this at the dock in Bacharach or not. But that simple task saves you about €4 off the Bingen > St. Goar cruise.)