1) This is painful. There are tools to help, but the most effective means I've found are having a policy to only commit notebooks in a reset, clean state (enforced with githook).
2) I don't understand. I've written full testing frameworks for applications as notebooks as a means of having code documentation that enforced/tested the non-programmatic statements in the document. Using tools like papermill (https://papermill.readthedocs.io/en/latest/), you can easily write a unit test as a notebook with a whole host of documentation around what it's doing, execute, and inspect the result (failed execution vs. final state of the notebook vs. whatever you want)
3) Projects like ipynb (https://ipynb.readthedocs.io/en/stable/) allow you to import notebooks as if they were python modules. Some projects have different opinions of what that means to match different use cases. Papermill allows you have an interface with a notebook that is more like a system call than importing a module. I've personally used papermill and ipynb and found both enjoyable for different flavors of blending applications and notebooks.