KARA.AEK
The Saver Day Pass is valid for any means of transport on that day. You simply hop on any train, bus, whatever, and could also simply take a previous or next service from the one you happened to select before buying that ticket… Which is the QR code you received by Mail, so there’s no need to link it to the app at all: It doesn’t matter which app displays that code, you could also print it on paper.
Buying a ticket as registered user is just simplifying things in case of a ticket refund. But with a Saver Day Pass, refunds are (almost) not possible anyway… that’s the downside of the cheaper price.
Oh, I forgot: No, it’s not possible to retrospectively add a ticket purchased as guest to an account.