Neville ChaimBerlin: ” Is what you’re saying actually mainstream? That the Mishnah Berurah was wrong about how most rishonim held and that were he alive today he would correct the “mistake”? Do you have a source that says this?”
Unfortunately, today one can’t make an argument, that the MB has been supersede, even with proofs. However, since you asked see the Bais Av (as cited by Gaon), and the following:
Even Yisroel:
http://hebrewbooks.org/pdfpager.aspx?req=673&st=&pgnum=44
Toldos Shmuel:
http://hebrewbooks.org/pdfpager.aspx?req=39121&st=&pgnum=337