Thanks for the extensive feedback!
We will most definitely take all this into consideration moving forward! 
The initial release of safe-app-android was put together to give Java / Android developers something to play around with and give their feedback. We will iterate over this to soon provide a better, more developer-friendly API. In fact, there is a plan to have simpler Rust libraries so that the API in all languages will be the same and super-easy to use.
Along with this we will most definitely work on improved documentation.
In the meantime, if you’d like to see the list of possible errors, this would be a good starting point.
For this there is already something available, you can run vault(s) locally and connect to them to test your apps in a local LAN setup.
This post should have all the information that you need.