All you have to do is take your device that has the vanilla app installed, open Facebook, and sign out of the FB account linked to your vanilla save. Then, sign into the Facebook account associated with the modded one. You will also probably need to clear your cache and/or storage for Hogwarts Mystery in your phone's settings. Launch the vanilla app and you should see the default opening cutscene, which you can skip. Then, click the lock icon and sign into your modded account from there.
You won't have any of your cheats available, obviously, so you'll just have to play through the Black Lake section normally. If you succeed in getting past the point where it crashes, save your game and then reverse the whole process on that device to reload your vanilla account.
The next time you open your modded game on the device with your modded account, you should be past the point where the crash was occurring. Honestly, it would probably be a bit less complicated if you weren't juggling a vanilla account in the process - you could just stay permanently signed into the modded account on both devices and switch between the two seamlessly, like the rest of us.