Hey @hjanuschka, I think @james will be able to better answer some of these questions but I can answer a few
We are using the Semantic Versioning so everything on from 1.0 to 1.9 will be backwards compatible. Version 2 will likely have breaking changes, we will be using the petitions site to guide us on what features we add and what features get moved out of the core and into a core plugin.
No, as we maintain completely compatibility with all plugins that support 4.9.x.
That is correct, we are only overwriting core files - you can find out more details here: Installing ClassicPress | ClassicPress Documentation
Technically, yes you could by overwriting the ClassicPress files with the WordPress files. We have had discussions regarding adding the ability to migrate back to WP within the migration plugin.