Nev,
“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?”
See the below link that states so in Anef Bais . Sefer Bais Av – He lived in the times of the MIshna Berurah:
http://www.hebrewbooks.org/pdfpager.aspx?req=2151&st=&pgnum=89
He seems to be one of the forgotten great gaonim in his times, he has an probation of the Minkser Gadol (who rarely gave any Haskamos – see below link) and the Sdei Chemed quotes him many times with highh regards.:
http://www.hebrewbooks.org/pdfpager.aspx?req=10002&st=&pgnum=3