I've thought about this. By Henry's will, the Katherine Grey is the first in line. She was born in 1540 (as was James VI). Mary takes power in 1553, Kate and Jamie are 13; Mary weds Philip and tries for children, but since: 1) she wants to keep England Catholic & 2) like her father, she was a sucker for a sob story (why H8 never saw anyone once he decided on their fate), she's going bring Katie to court and, from what I've read of her, Katie was more Cartland than Calvin - a romantic who spent time in the Tower for love. Mary will not wed her heir to the son of the Earl of Lennox and I suspect any one of my dogs would be a better King than Darnley. He's going to be 13 when Mary dies, another reason why that betrothal won't take place. Mary will wed Katherine to James to ensure peace since Parliament will not accept Philip wedding Katherine. James will be the lesser of two evils in this event.