Dear Śiṣya, Namaste.
You have asked some questions which would benefit many to get answered. With your permission, I am here displaying the my answer on the website.
- ॐ ऐं ह्रीं क्लीं चामुण्डायै विच्चै (om aiṁ hrīṁ klīṁ cāmuṇḍayai viccai)| is correct.
- When हुं (huṁ) is used with फट् (phaṭ) as the ending of the mantra then it retains a laghu उ (u) thus making it हुं (huṁ).