It seems to me that Young Ian is 5-10 years younger than Bree.
I also hate waiting for the next installment!
Well, as far as the question on whether the handwriting could be William's, you could go back and look at the letter he wrote to Jamie in The Fiery Cross when he bought the astrolabe at Lord John's request for Jamie and had it made in gold. Just compare the way the letters are written, the flow of them and see if they are similar. Might give you something to speculate further with, like maybe William has reconciled later in the story to the truth of his parentage and is the one to take care of the box of letters so it can find its way to Bree and Roger?