>impossible to reuse notebooks in each other... Or unit test them...
It's quite easy to unit test notebooks. Whenever you write a function you want to test, write some unit tests (or even just asserts) in the same cell it's defined, then any time the cell is run to define the function, the tests are run too. Likely you'll be doing some manual tests when you write a block of code cum function, and it's not hard to copy the input crafted from those manual tests to a unit test.
It's quite easy to unit test notebooks. Whenever you write a function you want to test, write some unit tests (or even just asserts) in the same cell it's defined, then any time the cell is run to define the function, the tests are run too. Likely you'll be doing some manual tests when you write a block of code cum function, and it's not hard to copy the input crafted from those manual tests to a unit test.