You’ve posted similar things here before; implying that there’s some big secret contradiction that only reveals itself with multiple viewings. The whole history of these type of murder mysteries on film are filled with endings that show or posit what actually happened, which often involve seeing earlier incidents from alternate angles or different points of view.
But there’s nothing in the solution of the mystery which ‘doesn’t jive’ with what we’ve seen before… it just fully explains it in a surprising but satisfying way. (You can bet that as a compulsive puzzle-maker Sondheim of all people would’ve made certain there were no loose ends.)
So are you implying that you think he DID? ie that you can poke holes in something he and Perkins wrote in the screenplay’s solution, that doesn’t make sense to you? That there’s a continuity error with the direction that you don’t think was intentional? Or are you just not familiar with the conventions and tropes of storytelling in this genre?
Feel free to mark with SPOILERS in your subject heading.