AAQ is right that congrary to Are R, we don’t find poskim overturning psak or inventing their own psak based on rishonim to go against established achronim. Are R also claimed that the MB follows kisvei yad as opposed to the chazon ish. The chazon ish wrote this explicitly, that it is against the belief in hashgocha that Hashem has given raboseinu ha’achronim the correct text of the rishonim. He says whatever rev akiva eiger had is emes, even if we find a different version. He says we can use them to augment our understanding of a sugya and shed light on things, like to answer a kasha on a tosfos, but not to overturn the established halachik decisions. Can you point to a place where the MB says anything different than the above?
Rav moshe feinstein often paskened like some rishonim over, say, the psak of the magen avrohom, but he didn’t do so because it’s better to follow rishonim – he did it because he learned the sugya that way and came out like those rishonim, but there were always lther achronim like him. It wasn’t as if for 500 years we’ve all paskened one way and then rav moshe came and overturned it because of rishonim…no one outside of MO would attempt such a thing (i.e. digging up rishonim to try and defend not covering hair). To their credit, when it comes to shabbos and kashrus, MO seems to follow normal halachik jurisprudence.
What AAQ said amounts to the rationale behind “halacha kebasrai”, that we follow the later opinions because they saw the earlier ones and paskened whichever way despite that.
When you want to know halacha lemaysoh, do you open up a rambam or a mishnah berurah? I should hope it’s the latter.
As for the discussion regarding hishtadlus, it’s not an issue of psak din. Hashkofa matters, while extremely important, are often decided on mesorah, including oral tradition. Actually, I’d argue that of all things, mussar and hashkofa are the most dependant on mesorah, because of not knowing when to apply which aphorism of chazal. Rav hirsh makes this point very well in the mishnah in avos “lo am haaretz chossid”, where he asks why it doesn’t say “tzadik”, since if he is unlearned he can’t even keep basic halacha, let alone be a chossid (who goes beyond the letter lf the law). He answers that we’re talking about someone who learned shas and poskim, but was not meshamesh talmidei chachamim. Chazal call such a person an am haaretz – so halacha he can keeo, and be termed a tzadik, but a chosid he can’t be, because for middos, hashkofa, mussar, etc…you absolutely need shimush to know how to apply daas torah to your life.