You’re right, the manual is out of date. As an independent (sole) developer, my time is pulled in many directions. The manual hasn’t been a priority. I have made videos for some of the newer features on YouTube (look for the Gentlemen Coders channel) (e.g., lens correction and fringe removal, etc.). That might be worth your time.
The term “smart albums” is a poorly-defined one unfortunately – Photos uses that term for a variety of albums, both system-generated and user-generated. System generated albums include media types (e.g., panoramas) as well as the RAW album. Those are accessible to apps. Photos does not provide user-generated albums to 3rd party apps, so there isn’t a way to display them.
As far as your wish list:
1. keyboard shortcut for crop – yes, that is something I’ve wanted to do for a while.
2. changing the crop – crop in general has a number of issues that need to be addressed. I agree that having to confirm with Photos every time is annoying. However, there isn’t a way to approve changes only once. Apple requires it for every image. It would be possible to add a “batch” feature where you approve a group of edited images at once, but that’s the best that can be done.
3. No, the app is not provided access to keywords or titles/descriptions from Photos. I have made formal requests to Apple and spoken to people there. So far, no luck. I have provided them some suggestions – we will have to see.
4. Photos does not support the idea of modifying the original externally (or internally for that matter) – it’s non destructive. You would need to use the external editing feature of Photos.
5. It’s possible for the app to make a duplicate, but the Photos library doesn’t have specific support for that. The app would need to download everything and then create a new image. That’s doable, except that because Photos doesn’t expose keywords or titles, the duplicate would not contain the keywords or titles. Because of that I have not implemented that feature. You would need to use Photos for that.