I assumed we would end up having the two essentially separate conversations. For me at least, the conversation about the historical mystery is the more interesting one.
It seems to me that Tey wrote with an agenda. While it is hazardous to do so, I am inclined ascribe Grant's opinions of historians to the author. It's the sort of opinion that outsiders of any speciality subject tend to carry; people are inclined to forget there are good reasons why subjects specialise. Historians may well be best advised to look to primary sources, but outsiders rely on specialists to provide useful, predigested, secondary sources so we don't have to spend the years that it takes to make qualified assessments of primary sources.
I don't like Grant, and I don't trust Tey to present the information in an unbiased manner. And I could argue: Why should she? This is fiction, the priority may well be entertainment rather than historical accuracy. I doubt whether Shakespeare had compunctions about adjusting history to fit with his plays (although it appears that Morton/More/Henry-VII have already managed to do some of this for him).
For the historical mystery, I think Shakespeare's portrayal of Richard III makes the whole thing all the more interesting and poignant. Now we can pick a side and feel as if we know what it is we are defending.
Philippa Gregory's version -
The White Queen - is turning out to be an entertaining read (I should be finished already but work has been getting in the way). But so far there has been little in it worthy of addition to our conversation here. (So far in my reading the second son has not yet been born - still a hundred pages to go. First person, present tense, would not have been my first choice for this, but it is an interesting way to view the history.)
For my part, I'm happy for people to share what they've found out about the historical mystery; I'm very pleased to learn I'm not the only one this stirred to wider research, but I don't have much more to say about Tey's book that you don't already know, or can't guess

.