|When I get the chance, I will read the essay on Boswell; however, wanted to comment on this first. It is pretty clear Shakespeare had to use a wealth of sources for a majority of his dramas and historical works. I am not as up on King Lear as I am on the histories or Hamlet, but followed a great deal because I recognise the sources that you cite in this. I take it you study Medieval literature, for some of the references go back quite a bit. I am not as up on my literary sources for the time period as my focus is history, though I admit much of the historical works of the 14th century have a literary feel to it (Most of Chaucer's work was intertwined as a mesh of fiction and non-fiction), but the medieval mind found it perfectly acceptable to mix a little fiction in with non-fiction. Makes my work frustrating, especially Jean de Froissart's Chronicles Siding with the English, he inflates casualty numbers for battles during the Hundred Years War to make himself look better in the eyes of the English.
That brings me to a point about Shakespeare in that many of his historical plays had to be modified to fit the prevailing mood of the time period, which was pro-Tudor and anti-Stuart/Lancaster/Plantaganet. Hence, when most people think of Richard III, they think of Shakespeare's characterization (or Sir Ian McKellen in the fine modern adaptation of the play), which was a little further from the truth. This is, of course, something not really applicable to King Lear, but indicitave though of the political realities in which Shakespeare worked. He had to make the play still look good for the government or it would be the last play he would write.
In all, I thought the essay was exceptionally well thought out and brought some interesting information on where and how Shakespeare might have gotten his sources for this play, and others.