The answer
The article cited in this edition of the Mighty Arrows Magazine is accurate. Below is another reference which corroborates the magazine version. God's saints and martyrs are dear and precious to Him. The underlying message that God gave to St. Mehrael in the words of the promise is that she will be able to intercede on behalf of petitioners that have special concerns. This is a great blessing bestowed on this saint in particular. If other promises were made other than what is written in St. Mehrael's biography, it is unknown to us at this time.
Please see the link as well: http://www.copticplace.com/Mohraeel.html.
Answered by His Eminence Metropolitan Youssef (opens in a new tab)