Most of the solution is already there? remove.

Make it clear that although the type of args can be any, they must all be the same.

Define what it means to sum up two None.

Maybe even leave the (None, None) testcase out, not sure how helpful it is.

At some places the ref. function doesn't use the same names for the input variables as the solution function, change to be consistent.

Exercises in the end are missing docstrings.

Exercise longest_sequence:

Part 1 already tests part 2 as well, I assume this is because of the naming of the exercises? in anycase an n^2 solution doesn't pass part1

I don't like the solution at all. I would show the solution with sorting first, which runs fine and is much better understandable than the O(n) solution, saving a log factor shouldn't matter for them.
If you really want you can have two solutions for part1 and 2 which show a nice improvement by the use of sets and hashing.

For part 2 I get no feedback when run, i could imagine this is again an issue with the naming of the exercise?

Exercise password_validator2:

Solution contains Counter function which is not explained.

Buckets reorginaziation:

Import necessary libraries (pathlib and ascii_lowercase, ascii_uppercase?) in the exercise cells.

Make the reference_solutions self contained. It should be possible to copy paste the ref solution and pass the tests. This also means the input variable names should be consistent