night train Italy-Munich--can we get of in Salzburg?

No law saying you can't get off any train early. You will actually be in Salzburg station for 12 minutes while they do a crew change. I don't know if couchette cars have attendants, so you will have to wake up on your own. Get your things together and be ready to get off the train when it pulls into Salzburg Hbf.

will all of the couchette doors open? Meaning that they do not group the Munich couchette travelers in cabins that do not open at intermediate stops?

If no one is around, you open the door by pushing the button or pulling the lever. That is standard practice on European trains. Whoever is standing by the door opens it when the train stops. It is locked until the train stops, then automatically unlocks when the train is completely stopped. I've seen people hold the button down as the trains is slowing down so that it will open the instant it unlocks.

Thus if nobody is getting off at a stop, or nobody is boarding that car at that stop (the same button is on the outside of the carriage), the door never opens. If you are standing by the door when the train stops, and don't push the button within about 2 seconds, somebody behind you will reach over your back and push it for you. Kind of like that horn honk when the light turns green and you don't immediately stomp on the accelerator.

Don't worry. The cars to Munich are not reserved exclusively for travelers to Munich. Every traveler whose destination is between the Austrian border and Salzburg must either be put in a carriage bound for Munich or one bound for Vienna, and s/he must of course be able to leave the train at the desired destination (in that case Villach or Salzburg).

You cannot, however, use your ticket for a later train to Munich since it is train specific.

You should be aware that the train attendant will collect your passports after boarding and return them to you the next morning. That has been my experience as recently as August 2017. It has usually been returned at breakfast, sometimes a little after.
I would be cautious about getting off early for this reason. If the passports haven't been returned, it is difficult to track down an attendant.

You can always purchase a Bayern ticket for cheap Salzburg to Munich trains.

You should be aware that the train attendant will collect your passports after boarding

That's still true but you just tell the conductor you want to get off in Salzburg.

thanks. Stuff like that was what I was wondering as well. It would be 6am, so not an easy hour to find someone. Is is OK to tell them we are getting off sooner? Is that officially allowed? It's not like ditching a layover city on a flight? Does getting off in a different country matter?

You will not be the only one getting off in Salzburg and the conductor must bring the other passes anyway. Just tell him in the evening that you've decided to leave the train in Salzburg, so he can sort in your passports correctly. The tariff system does not work like the plane. Finally, you travel within the Schengen area, so nobody will care whether you get off in Salzburg or a few miles later in Bavaria. If it calms you, then write to the ÖBB customer service. Have a nice trip.

Your passport will be collected at the start of your journey because 1. you are in a couchette and 2. crossing an international border.

I took a night train going from Germany to Austria ie, Frankfurt Hbf where it finally left at 2350 hrs) to Vienna. My passport was not collected since I was in a 6 person compartment, where there were 5 including me sitting. I've never had my passport collected when crossing an international border in a night train because I was not in a sleeper or couchette.