Does the pre-marital mutual thigh-fondling (mufakhadha), without penetration, that occurred between the fiancé and fiancée before the marriage contract (aqd al-nikah) invalidate the marriage contract, and does it necessitate a waiting period ('istibra)?

You must repent to Allah Almighty for what happened during the engagement period, because she was a stranger to you. Your marriage is valid if no adultery occurred, and she is not required to observe 'iddah (waiting period). You must ignore these whispers (waswas) and consult a psychiatrist.