Hi !
Am I allowed to add/modify the test cases of one of my contributions, even after it has been accepted ?
Yes you can, as far as your changes would not invalidate already passed submissions, would not fundamentally change the difficulty or nature of the puzzle.
I am not so sure about the point on invalidation. In my opinion if the new cases still follow the original statement and constraints, it should be fine to add. If some submissions originally pass because of insufficient test coverage, the test coverage should be fixed.
For bug fix or improvement purposes, I think it should be done even knowing that it can break some existing (and imperfect) codes, for the benefit of clearer requirement definitions for new codes to be submitted.
Yes, you can but leave a message in the contribution’s page.