Neville ChaimBerlin: “I don’t think the Mishnah Berurah is contradicted at all. He allowed the 600K shittah, and just said it’s a nice thing to hold by the stricter one. I don’t think anyone here has denied that if an individual wants to hold by the machmir shittah, it’s a good thing. Enforcing it on an entire town is a completely different story.”
While I agree with your basic premise, it is more nuanced than you are stating as fact. The Mishnah Berurah is following the Mishkenos Yaakov, who clearly maintained that one cannot rely on shishim ribo at all. The MB realized that the minhag is to rely on the criterion, thus he suggested that a baal nefesh should be stringent. My argument is that the main reason why the MB upheld one should be machmir was because of the MY’s argument that the overwhelming majority of Rishonim did not accept the criterion of shishim ribo. However, we now know that the opposite is true, and that all the Rishonim of Tzarfas and Ashkenaz upheld the criterion of shishim ribo. Therefore, even the MB should/would admit that a baal nefesh can rely on the criterion.