The solution I found so far is:
1) Play using the original game until you pass the tutorial part (so as soon as you can do whatever).
2) Enter inside the Shadow Realm part. Everything should be okay, and if asked, you should be able to use Google Play to connect your account. Then go to your profile, and click on Save just in case.
3) Use ZArchiver. Go to the folder "Android", "data", and rename the folder "com.ondi.fortias.saga" into something else (like "com.ondi.fortias.saga_"). The full path to the game folder is "/storage/emulated/0/Android/data/com.ondi.fortias.saga".
4) Uninstall the original Fortias Saga app.
5) Install the modded version.
6) Before opening the application, use ZArchiver again, and restore the name of the folder of the game (if it was "com.ondi.fortias.saga_"m change it back to "com.ondi.fortias.saga").
You should be able to access the modded app with your created account, and the Google Play thing should be gone because you already linked and logged into your account.