What is the legal ruling for someone who utters an inappropriate word about the Prophet, peace and blessings be upon him, spontaneously and unintentionally? And is this considered apostasy necessitating re-entry into Islam?

It is sufficient for the questioner to seek Allah's forgiveness and repent to Him, and to guard his tongue except for good. He should know that declaring someone an unbeliever is not a light matter, and that one whose Islam has been established with certainty does not lose it through doubt. Not everyone who falls into disbelief has disbelief befalling them. The condition the questioner complains of is a whisper from Satan, so he should seek refuge with Allah from the accursed Satan, turn away from these whispers, and devote himself to obedience to Allah.