Using the ticket requires to use a phone with either Google Play or an iPhone. Alternative phone firmware (degoogled Android, SailfishOS, etc.), paper tickets, smart cards or anything else are not supported.
From the seller? It wasn't when I checked many months ago, when they had special Interrail campaign with flexible dates for app users but only fixed dates for paper tickets.
Of course there are unofficial sites. But firstly I would not trust that their APKs are unmodified and secondly I would not rely on it that it works with a ticket for 300 Euros or so. Some tamper/rooting detection might still trigger in an alternative firmware. Well, the youth ticket doesn't cost anything, but see above why that's not an excuse.
