I think, in order to break up the curse, two must fallen in love with each other. BECAUSE If only one is to fall in love then he will fell out of love. From what I heard so far, mermaids can only fall for one person which means if his or her love is not returned then it will bring them so much pain that it turns them into a sea foam.
I'm not sure....
....but maybe the reason why Isaiah and Gabriel has different eyes is because it was given to Adam when he became blind.
Oh i just notice after u said Adam do have the same eyes colour as them