Bayern Ticket

Kahl is the first station in Bavaria coming from that direction. Even using ICE trains it is about 8 hours journey. I see tickets in march as low as 49 euro. So taking slower trains with more connections could stretch that to 12 hours or more. How many in your party? A QDL ticket might work just as well.

Yes, you would need a Quer-durch-Land (QDL) ticket. It takes nearly 12 hours and is 5 changes minimum. The fare would be 44 euro for the first person and 8 euro per person each additional person up to 5 total traveling. The first train you can board is at 9:26, but the 10:58 is the best to get you to Salzburg by 10:30pm.
If you travel on a Saturday or Sunday you can leave anytime. Change trains at Koblenz, Frankfurt, Wurzburg, Treuchtlingen, and Munich. All the connections are comfortable except Treuchtlingen, which is 5 minutes, but it is a small station, so you should make it. The alternative would be a nonrefundable advance purchase ticket with a single change in Koblenz taking 8 hours and cost 129 euro for 2 (224 euro for 2 is the walk up price). So there you have it, for 2 people, 52 euro, 129 euro, 224. They all have their good points and bad points.

If you book ASAP (up to 92 days allowed) on bahn.de, you can get a Sparpreis fare as low as €89.00 for two people traveling together for Cochem-Koblenz-Salzburg.

Thanks for your replies. I have looked at the QDL ticket and understand we can only use regional trains. How do I book a ticket with this pass? When I enter the cities, it brings up fares using the ICE trains and regional trains combined.

If we decide to try and get a Savings Fare, how far in advance can we get that ticket?

To eliminate seeing the ICE trains, un-select the 'prefer fast connections' box

Savings fares are available 92 days in advance

The sparpreis ticket Tim found is the rock bottom price for an advance purchase nonrefundable ticket. As he said, it will go on sale 92 days before your travel date. 92 days from now is the May Day holiday period, and all of tickets for this price are already gone. The ticket shown was for Tuesday April 29. The point is, they sell out fast, and then only higher priced tickets are available for sale. So start looking 92 days before your travel date a www.bahn.com. You can print the ticket at home, but remember to bring that credit card with you when you travel because that will be your ID and proof that you are the person who purchased it.

The QDL ticket can be bought anytime before you travel. They are cheapest when you buy out of a vending machine at the station. I am not sure if a Pin & Chip credit card is required, that is the case in France. There is a 2 euro service fee if you buy from a ticket agent.

edit- Actually check the "local transportation only" button on the menu to show the QDL connections.
Interestingly, if you check the "all without ICE" button, you uncover a sparpreis ticket with 3 connections and 9 hours travel time for 49 euro for 2 persons total leaving Cochem at 8:58 am. It also shows you can travel 1st class for 59 euro total.

The Bahn automats were accepting strip cards last fall when I was in Germany.

The Bahn website (German language side) has an "Automatentour" under Services > Vor der Reise > Ihre Wege zur Fahrkarte > DB Automaten. The website's tour is in German, but the actual automat can display in English (touch the British flag). The opening screen shows four square touchpads. The touchpad on the upper-right, labeled Gesamtes Angebot in German, is where you touch if you want to purchase a QdLT or SWT. That gives you eight new touchpads in two column. The second pad down on the right is for Gruppenkarten (group tickets, QdLT and SWT). Regional tickets, like the Bayern-Ticket are on the top LH pad. The part of the tour on paying with a credit card specifically shows to insert the card with the strip down and to the right, so I assume they use it. And, of course, the automats take cash.

I don't see anything on this thread or on her other two threads about the day of the week or number of people. The QdLT (Quer-durchs-Land-Ticket) is valid on workdays (44€ 1st person, 8€ each extra person to 5 total). On Sat, Sun, and holidays you can use the Schönes-Wochenende-Ticket for 44€ total for 1 to 5 people. Same limitation of regional trains, but no 9 AM restriction.

For Savings Fare tickets, the first person is x9€; each additional person up to 5 total is x0€ (i.e., 49€ for the first, 89€ for two [49€ + 40€], 129€ for three, etc). Children under 15 are free with their parents if declared when booking the ticket.

There will be five of us. Haven't firmed up the dates yet, so not sure if we'll be making this leg of our trip during the week or on the weekend.
You've pointed me in the right direction. Thanks for your help.
Now reading Rick Steve's book, trying to decide if we should base in Cochem and go to Bacharach as a day trip or spend two days in Cochem and then move to Bacharach. I think that's what he suggests.

If you stay in Cochem for two days, then move to Bacharach for more days, you have the extra time of packing up, checking out, checking in, unpacking, but if you stay in one or the other, you have commuting time, including a change of trains in Koblenz. I guess it's something you have to decide.

On a trip I started to plan recently but never took, I was going to stay midway, in Braubach, to cut commuting time to Bernkastel and Bacharach/St. Goar (and, of course, be right in town to visit the Marksburg). You'll find a list of hotels in Braubach here. I've never stayed in any of them, but I did have lunch in Wieghardt and thought it looked nice. From Braubach, it's about an hour to Cochem and 20 minutes by train to St. Goarshausen, from which a ferry go across the Rhein to St. Goar.