Clear room implementation is *not* required.
The idea of a clean room implementation is to remove the possibility of the resulting code being in violation, however, that's not actually required to avoid being in violation. That just makes it much easier to show good faith.
If you implement a test suite, and then have the AI generate a version that complies with the test suite, it's entirely possible you are not in violation.
Symantec code validation would be prudent, but not necessarily required.
(Replace all variables with generic references and see if the code matches up).
Keep in mind, have a dozen lines of code actually match still doesn't mean those lines are a violation, it just means they could be.