score:7
Yes, there's an "MVV App" which allows that. I am not sure what payment option is best for you; but you can just download it and check if it has something that works for you.
You do not need to print or scan it, you just buy it at least a few minutes before the train goes and make sure you don't lose the Smartphone (or run out of battery...). There are no gates in the Munich transport system; there are very rare checks (by humans), and they will then just ask you to present the ticket on your smartphone. Note that it's usually necessary to buy your ticket before you enter the actual platform. There are signs for that (in German at least, can't remember if they're in english, but then it's also pretty obvious by the way everything is structured).
That said, getting the ticket from one of the vending machines is probably easier for you. Get a day ticket ("Tageskarte") which gives you access to the inner district (which contains the stations you mentioned) for 6-7β¬ per day in 2018, which is likely less than you would pay if you get individual tickets for each single ride, if you go multiple times a day. And if you are more than one adult and stick together, then getting a group day ticket is best.
Note that the MVV (city train network) tickets are very "light-weight", i.e. they are not personalized, are not directly fixed to a specific train, are not related to the regional/long-distance trains and so on.
Between Rosenheimer Platz and HackerbrΓΌcke, any S-Bahn is the correct one as this is the completely linear part of the Munich S-Bahn. You only need to check the direction. If you look at the network plans hanging around everywhere, you will see that the central station is between those two stations. And at every platform, there will be signs that tell you whether you are standing on the side that goes in the direction of the city center (correct) or outwards (incorrect for you).