I think you're all clutching at straws. I wonder, instead of "Mandela Effect", if there should be a term to describe a phenomena where a mass of people can
think they've seen something purely because it somehow makes more sense that it
should have been played out that way? Yes, it seems somehow logical that Dolly should have smiled to reveal a mouthful of metal, but all evidence points to the contrary.
I've skimmed through magazines from the period which contain reviews and stills: Starburst, Starlog, etc. Looked at the trading cards released in conjunction with the film, and all photos of Dolly show her sans braces. Even Mad magazine, which would surely have taken the braces trope and run with it, have drawn Dolly without:
Mad magazine #213, March 1980
As for just lower braces... again, this is disputed by stills:
Even the actress, Blanche Ravalec, has commented: "No, Dolly has never worn the braces. It was never intended for that matter".