The melcha is a private family affair and the ruling family would never make an official announcement about it. However, the marriage has been confirmed by posts on Sheikh Majid's forum, and by Instagram and Twitter posts by known friends and family members of the couple. Also, photos have been published of the event by Hessa's sister and a close friend of Sheikh Majid's.
Melcha = marriage. You would have to divorce to break it. As far as I know, Sheikh Hamdan was only engaged. I never heard any reports of him signing a marriage contract with Sheikha Sheikha.
Perhaps Majid learned from his brother and kept his engagement to Hessa a secret until they were sure that they would marry.