[ballenf]

I think an even better approach would be to have the concept of fixed tiers of locking combined with evicting the decryption key for any Lock Screen above the basic PIN.

And you can only move down one tier of unlocking at a time. Unlocking SIM PIN moves you down one tier to phone PIN screen.

[qwertox]

I'd go for one screen with a queue of prioritized unlocking tasks which need to be completed successfully one after the other. These tasks could get the chance to hand over a fragment which the screen embeds and presents to the user, in order to properly modularize the tasks.