At the time, the Governor General was in Africa on a state visit, I believe. I doubt that the letter, as a physical entity, was conveyed to her; I have no doubt that the contents were. I have absolutely no doubt, none whatsoever, that at the time the Governor General made her decision in 2008, she was fully aware of the contents of that letter.
I've written about this. Rightly or wrongly, I believe that a main consideration that she took into account--and I say “I believe” because I don't know--was the viability of the coalition expressed there. It was quite different in Ontario in 1985, because there was very little doubt that the two opposition parties would support one of them in government for a time. You can remember that there was a very effective argument, though constitutionally not a terribly enlightening one, from the Prime Minister's side that he was elected Prime Minister and that this would be an illegitimate government.