Firstly stop worrying about maybe’s.
Your app needs to access to internet every couple of days but can be operated without access to Internet. So just make sure if you are in a wi-fi area or have inexpensive data on your phone you access it. Start the app and it will check and update in the background.
The next thing is obvious - protect the master phone with simple practices - protective cover, don’t leave it on display and keep charged.
In the event you lose your phone make sure you have a second phone with the app installed. Also make sure you have a record of your pass details and the contact e-mail or website link for customer services.
Immediately contact them and include travelling now and your pass numbers in the title. They will then remove your passes and you will be able to add them to your second phone.
It is strongly advised that you do not load trains from your trip into your pass in advance in case you want to cancel a travel day.
It is highly unlikely that a print out of your QR code would be acceptable, especially if you change trains. The pass displays the full ticket details (Dep and arrival stations and time) to complement the QR code It would also be easy to copy and hand over to many friends.
