German Train Pass details

I would not do a German train pass. For the day trip to Salzburg from Munich, you can use a Bayern Ticket which is 25 EUR for the first person, and 6 EUR for each additional person up to 5 traveling together. So that will be 43 EUR total for the 4 of you and and you are good for the day. You must begin travel after 9 am on weekdays and take only regional trains. You can travel anytime on weekends. There is even a slightly cheaper option for sale by Meridian trains, which operate the regional trains between Munich and Salzburg, called Guten Tag Ticket.

For Munich to Berlin, buy tickets now which are nonrefundable, except with a fee. I see ticket prices from 57 to 90 EUR per person on a random June weekday. For Wittenberg to your exit airport, do the same as soon as you have your airline tickets bought. Wittenberg-Lutherstadt to Berlin can be done with a Regio 120 Ticket for 60 EUR total for the 4 of you. similar restrictions to the Bayern Ticket. There is also a Brandenburg-Berlin ticket available for certain trains as low as 29 EUR total for 4 people.

Two 2nd class twin Flex GRP's would cost you 584 EUR, you can do better than that using Sparpreis advance purchase tickets and the Bayern Ticket.

Thanks.

Yes, after I posted I went back to the DB website and dug a little deeper and found point to point tickets to be cheaper.

For the Munich -- Berlin ticket would that be train specific for the lowest cost? If so, how tricky is that to figure out at the station?

If you buy now on-line a Sparpreis ticket, it is train specific. It is just like an airline ticket, get to the station 20 minutes ahead of time, check the board and see what track your train is departing from, go there and board. In fact, airlines are way more complicated than that and no one seems to have a problem with it. You don't have to show up 2 hours ahead of time, don't have to check luggage, don't have to go through security. You will have your ticket in hand, because you printed it out on your home computer when you make the purchase. If you want seat reservations, that is another 4.50 EUR per seat, otherwise you just find seats that have not already been reserved (electronic indicator above the seat).

Sorry, yet another question about the train specific tickets -- what happens if the schedule changes or the train is cancelled for some other reason? When I was Europe before and had a more flexible ticket, the schedule changed from when I made a tentative plan in the evening to when I tried to catch the train I'd planned the next morning.

If a cancellation happens, Deutsche Bahn will get you on the next train, If you had seat reservations, they will make you new ones. You can also cancel and rebook up to the day before travel for a 19 EUR fee, plus the fare difference of the new ticket.
https://www.bahn.com/en/view/offers/germany/saver-fare.shtml?dbkanal_007=L04_S02_D002_KIN0060_ST-SPARPREIS_LZ01

The schedule should not change, as you have a train specific booking. Deutsche Bahn will know when you buy a ticket what the schedule is. There is system-wide schedule change due on June 11, but DB already knows what the new schedule will be when they sell it. Now there could be a mishap on the line that will cause a delay, but they can reroute you, probably with a connection to get you to your destination. They aren't like airlines that will cancel a flight if it is not selling well, or make monthly schedule changes. Once a train is on the schedule, it stays there for 6 months.

Perfect. Thanks -- I sorta figured that, but wanted to check first.

Agree with all the ideas posted above. I order my point to point tickets in advance from the Deutsche Bahn website and they arrive in the mail within a few days, which is amazing since I live near Vancouver. Seniors get reduced rates after age 60, and this allows me to travel my Zagreb to Munich route (via Salzburg) in first class for very little extra. First class and quiet zone seats on InterCity inside Germany are awesome. Second class is full, but comfortable.
For family groups the Regional Bahn offers are great, just remember these are slower, local trains. Also, when boarding any of the Inter City trains in a main station make sure you are in the right train - some platforms are long enough that they have two trains leaving in the same direction at the same time. Some frantic last minute running happens when you can't find your reserved seat in the correctly numbered compartment - you are on the wrong InterCity train! Check the one in front or behind. Also every platform has a helpful map, as Rick notes, to tell you exactly where your train compartment stops (platforms have overhead signs with ABCDE. Inter City trains tend to leave exactly on time, and connections are deliberately tight. Train staff speak excellent English.
Munich station has a central Info centre that solves problems - all staff speak English. It is usually full of foreigners trying to figure out similar issues. Take a number on entry and watch 180 degrees for the first available helper, to get your turn.

Thanks!

One more question about the regional trains -- if we buy the one for Bavaria are we restricted to which train we get on? I know it has to leave after 9:00am, but we don't need to worry about finding the cheapest one right?

Must use a regional train. The ones to Salzburg are operated by Meridian Trains and go from Munich Hauptbahnhof at 55 past the hour every hour, with a few exceptions here and there.
The first train you can use during a weekday is at 8:55 am, but that is before 9:00 am. The solution is to purchase a local transit ticket for the Munich Inner Zone for 2.80 EUR. The train stops at Munich Ostbahnhof at 9:03 within the Inner Zone, so now the Bayern ticket is legal. Of course, if you are somewhere else in Munich, then just travel to Munich Ostbahnhof and get the train there.
These trains are unreserved, just find empty seats. Basically, they're commuter trains.

"After a week in Wittenberg, we will need to take a train to either Berlin or Frankfurt."

If it's Berlin, the 4 of you can make this trip for a grand total of €29 with a "Brandenburg-Berlin Ticket" (day pass) bought from a ticket machine in Wittenberg on the day of travel. The direct Regionalexpress (RE) trains get you to Berlin's main station in 1.25 hours. This day pass is valid anytime on Sat or Sun, after 9 am weekdays. Details on this day pass and the Bayern Ticket are available HERE.

If it's Frankfurt, you will probably want to look into a pre-purchased Saver Fare on the long-distance (high-speed) trains.