So, it seems more likely that Quirrel was behind the plot.

The thing about there only being seven houses seems big, though, and as far I can tell isn't from canon. (The list of purebloods, for example, doesn't include Jugson, though 500 years old might not be enough to be Most Ancient. I think we have HPMOR confirmation of Malfoy, Potter, Greengrass, and Longbottom, and I think in canon the only ones that get that description are Malfoy, Black, and maybe Potter (really, Peverell).

The 1926 hint narrows it down to four canon characters (though, of course, Bones might be mistaken). Interestingly enough, all of them were sorted into Slytherin- Tom Riddle, Rosier, Avery, and Lestrange. All of them were Death Eaters, and so it seems most likely it's Tom Riddle. (He would be the last of the female line of the Gaunt family, descended from Salazar Slytherin, which seems like it qualifies for Most Ancient. But I suspect the female line doesn't count for things like the Wizengamot, in canon at least.)

(Interestingly, in canon, Morfin Gaunt was memory-charmed to believe that he was the murderer of Voldemort's parents. Riddle did that to cover up a number of his murders. Even more pieces falling into place.)

Tom Riddle as hero seems... really bizarre, though. Who was Voldemort instead? (It seems implausible that Voldemort could have been an alterego; I suspect quite a bit of his pureblood support came from his lineage.)