What is the commentary on the difference between the two readings of "Maliki Yawm al-Din" (مالك يوم الدين) and "Maliki Yawm al-Din" (ملك يوم الدين) mentioned in the question, and what is the ruling on using the phrase "wal-salamu 'ala man ittaba'a al-huda" (والسلام على من اتبع الهدى)?

Al-Qurtubi mentioned that the phrase "Malik" [King] is more eloquent in praising the Creator, and "Malik" [owner/possessor] is more eloquent in praising created beings. As for the phrase "and peace be upon whoever follows guidance," it appears in the texts of revelation as an address to non-Muslims, such as Musa's address to Pharaoh, and the letters of the Prophet (peace and blessings be upon him) to kings. It is not for greeting and supplicating, but rather it means safety from Allah's punishment for those who embrace Islam. Therefore, a Muslim should not address his Muslim brother with it, as it contravenes the Sunnah and what it might imply, namely accusing the other party of not following guidance.