"Do you think we would miss much if we disembarked at Bacharach vs continuing on to St Goar?"
Yes, about half of the best part. Bingen to Bacharach is 15km. Bingen to St. Goar is 28 km. So Bacharach is roughly the mid-way point for the cruise segment which Jo, Sam and I have all recommended. (You're dealing with that "I booked in Bacharach" struggle that we discussed earlier when you were choosing a place to stay - it's just plain trickier from there to do this cruise segment.)
Am I correct that you arrive on June 6 by car in Bacharach for 3 nights?
"The 14:15 steamboat out of Rudesheim sounds like the way to go."
Wrong time... to ride THAT boat you will need to depart Rüdesheim at 16:15 or Bingen at 16:30.
It would probably be wise to split your cruise onto two separate days... June 6: Your driving time to Bacharach is less than 3 hours; it should be pretty easy to park, check in or drop bags... then take the 20-minute train ride at 15:36 to Bingen Rhein Stadt. Then walk a few minutes to the cruise dock... After the 16:30 steamer cruise, you'd be back in Bacharach at 17:15. Then... on June 7, re-board a KD boat in Bacharach and continue your cruise north to St. Goar or to Boppard. First boat north from Bacharach is at 10:15. You'll reach Boppard at 11:50 (or St. Goar at 10:55 - but it makes sense to cruise on to Boppard since that's on your sightseeing plate.) TICKETS: Buy on June 6 for the whole cruise segment (Bingen - Boppard.) When you disembark in Bacharach, just be sure to have the cruise personnel authorize your cruise ticket for next-day travel so you can finish out the segment you paid for. After Boppard, catch a direct train back to Bacharach.
"Is there any reason at all to go all the way up to Koblenz?"
Generally no, not by cruise boat.
Rüdesheim: You can drive... cross the river by car ferry just south of Bacharach in Niederheimbach. I would suggest a tour of Marksburg Castle in Braubach as well while you're on that side of the river. It's an outstanding, intact medieval castle.