Well, clearly there'll have to be some re-writing involved for them to include elements from all the routes, because you're right that the routes don't overlap much. Beyond just the different writers, most of the routes are heavily romance-dependent, so they can only be merged up to a certain point without being rewritten. That being said, I really can't imagine them going for Sakuno's route without a massive re-write, as despite Sakuno's character prominence, her route isn't very connected to the main theme of the work or that opening narration. (It would actually annoy me somewhat if they did Sakuno's route given this opening; it'd be a bit like a certain other eroge adaptation with similar original character designs which was heavily modified from the original PC game.) So anyway, since I'm barring total rewrites for the moment (the first episode was fairly faithful after all), I can only go with either Airi or Miu, and I think most bets are on Airi. I think Sakuno will have one episode later where the go into the first part of her route (with the flashback, to tie back to the opening episode), and then sort of reset to the status quo. Wouldn't surprise me if that happens just before the Airi ending.
So my prediction would basically be:
Episode 2: Common route (rest of the introductions)
Episode 3: Common route + start of Miu branch
Episode 4: Common route (Airi's home discovered)
Episode 5: Pets Club (Miu & Sana backstory, threat to close the club)
Episode 6: Pets Club (resolution)
Episode 7: Ange One-Off?
Episode 8: Return to Airi branch
Episode 9: Resolution for Miu/Sana (disappearance of Pannya?)
Episode 10: Sakuno One-Off
Episode 11: Airi Confession
Episode 12: Airi Denouement
Of course I could be totally off, and some of the content could be shifted up and down a bit, but it was fun to sort of think it through.