I have used these tickets lots of times, with local train connections to weird places like Meiningen. I went into it thoroughly at one point, and remember learning that the Regio, S-Bahn and local trains were exempt from the obligation to use only the trains indicated on the booking. So I reckon it is completely legitimate to hop on the S-Bahn to the airport when you arrive without penalty.
I remember also learning that the Swiss portions of the same ticket were also completely exempt from travelling on the booked trains. So that meant I could stop in Basel, see friends, even stay the night, and continue later (within 1-day) to Montreux or wherever.
Also, I think that the whole stretch could also be completed, within the time limit of 1 or 2 days or whatever it is, using any trains than IC or ICE, obviously the slower ones. So I could have stopped in Mannheim, then continued to Meiningen by Nahverkehr (slow local trains). But I never did this, though.