I am in complete agreement with Mr. Winter. As I once stated on these pages, the designation of "mezzo soprano" didn't seem as definite during the times of Bellini, Rossini, and Donizetti. In truth, the legendary Maria Malibran was more of a "mezzo" in the manner of Joyce DiDonato is today. Among Malibran's operas: Maria Stuarda, La Sonnambula, I Puritani, Norma, La Donna del Lago . It was only in the very recent past that there were "Malibran versions" of these operas. Cecilia Bartoli recorded the "Malibran version" of La Sonnambula", and Katia Ricciarelli recorded a "Malibran" version of I Puritani. The big three bel canto composers were willing to make accommodations for "lower voiced sopranos" who took an interest in their operas, so they sanctioned and approved of slight modifications. There were no, to my knowledge, no key transpositions, but the sopranos like Malibran and more recently, Gencer, Caballe, and now DiDonato merely resolve the arias, duets, quartets, and choral endings downward. It is highly likely that had Maria Callas gotten to Maria Stuarda , she would have sung it in the same way that the above mentioned artists sang it.
When Joan Sutherland and Beverly Sills undertook this opera, they happened to have the stratospheric extensions to their voices where they could resolve all the music upwards into the areas of high D and high E flat. Both made some spectacular effects with these notes, but that didn't stop Montserrat Caballe' or Leyla Gencer from making their own magic by resolving all the climaxes downwards . We all know that Montsy never had a top E flat in her entire vocal life. Gencer possibly at one time, but certainly not at the time she was singing the three Donizetti Queens. When I heard Janet Baker's Mari a Stuarda, I too had mixed feelings about it, having already heard Sutherland and Sills doing all the upward extensions. Baker's singing was great, though she was certainly no bel canto singer. Still, she sang the role exactly in the same keys as Caballe, Gencer, and now DiDonato. She simply resolved all the set pieces downwards to the final note an octave lower.
I remember so distinctly when Lyric Opera of Chicago in 1969 presented I Puritani with Alfredo Kraus and Adriana Maliponte. I was in my early twenties and had been weaned on the Callas and first Sutherland recording. Both of them made use of their vocal extensions to high E flat, and that was the way I was used to hearing it. As it turned out, Maliponte, who was a very fine soprano, simply had no high E flat in her voice. When she landed on the upper B flat, she resolved it downwards to the lower E flat rather than to the high E flat that Sutherland and Callas interpolated in their respective recordings. The critics made no note about it, and found it strange. So I obtained the original scores from the Music Library at Chicago Musical College at Roosevelt University and did some research.
I examined the Puritani score and lo and behold, I found no E flats anywhere (though I did find that insane high F for the tenor at the end near the end of the opera!). Then, Mirella Freni did a recording of Qui la voice & the cabaletta --- and SHE sang no top E flat either. She sang a beautiful and towering high B flat (the cabaletta is written in the key of A flat), and then simply resolved the piece on the A flat just immediately below it. It was a perfectly acceptable performance, and this is the way she sang the opera with Pavarotti at La Scala in 1969 under Riccardo Muti.
Rossini wrote most of his operas for Isabella Colibran, who sang Rosina, Isabella, Cenerentola, La Donna del Lago, and BOTH Arsace and Semiramide (on different evenings, of course.) In addition, in Donizetti's original version of Lucia di Lammermoor , Lucia is given no E flats either. Montserrat Caballe did a rather controversial recording of it in 1976, but the fact remains that she sang it exactly the way Donizetti wrote it.
Bottom line: A "mezzo" soprano like diDonato, who has a fine upward extension to high C, is perfectly entitled to sing Maria Stuarda. Sutherland, Sills, and to some extent, Maria Callas, were in a different vocal arena altogether.