I think it's more likely that the Lady Mary Fitzwilliam in question is Lady Mary Butler, born Mary Grace Louisa Butler, daughter to John Butler, 2nd Marquess of Ormonde and his wife Frances. She was born in Kilkenny, Ireland in 1846 and died in Malton, Yorkshire in 1929. She married on 11 July 1877 the Hon. Henry Fitzwilliam, the brother of the lady discussed in the prior posts, who was the second son to the 6th Earl Fitzwilliam, and also an M.P.
Unless I'm mistaken, as the daughter of a marquess who married a lesser son of an earl she was entitled to retain her first name in her courtesy title after her marriage, so she was styled Lady Mary Fitzwilliam rather than Lady Henry Fitzwilliam, although I admit I'm a little confused regarding this courtesy usage since he was also an M.P.

There's a photo of a Lady Mary Fitzwilliam facing page 152 in the memoirs 'Looking Back' by Sir Seymour Fortescue. Taken onboard the HMS Surprise, included in the photo is the Duke and Duchess of Edinburgh, and it must date from 1886-87, the years that Seymour was First-Lieutenant of the 'Surprise'. Although I can't be certain that this is the Lady Mary the rose was named for, the fact that the family had close royal social connections makes it likely in my opinion - Lady Mary's mother the 2nd Marchioness of Ormonde was lady-in-waiting to Queen Adelaide from 1844 to 1849, and Queen Adelaide was the godmother to Lady Mary's brother James (who became the 3rd Marquess of Ormonde), and Lady Mary Butler herself was one of the bridesmaids at Princess Louise's wedding in 1870.

I note that the rose 'Lady Mary Fitzwilliam', although released in 1882, was already exhibited in 1880 under that name which is only 3 years after Lady Mary Butler's marriage. I'm not sure if there is any personal connection to Bennett but the timing at least is fitting.
 
There may still be some other Lady Mary Fitzwilliam possibility out there but this is the most likely candidate, in my opinion.