NC
I’m sorry I dont follow. The M”A says beferish, (and he is quoting earlier sources) “Minhag Oker halacha” That seems to be a pretty good proof that minhag overrides halacha. Granted minhag doesnt mean any act under the sun it has to have SOME halachic basis but if it does the halachic consensus (i.e. the normal way halacha is decided) can be ignored. In other words the concept “Minhag oker halacha” clearly exists!
furthermore although some seem to be arguing that it doesnt exist, nobody has provided an example of a minhag that DOES NOT trump halacha. Even the isolated examples brought where some say “No minhag doesnt trump halacha” (nusach sefard, sukkah on shemini atzeres) we still dont “write off” those who “ignore” halacha and follow thir minhag.
To sum up:
1) it is a Magen Avraham “Minhag oker halacha”
2) In practice nobody has provided a minhag that is not oker halacha, so by default it is (broadly speaking, again there are excellent examples where isolated Rabbanim say to ignore minhag in place of halacha however none of us view those who eat indorrs on Shemini atzeres as those who dont eat matzah on second day Pesach or those who daven sefard as not having davned today! (as an aside when I was in yeshiva my mashgiach gave a lengthy vaad on why kesav beis yosef was the real kesav and kesav ari shouldnt be used. I asked him afterwards my tefilin are kesav ari what should i do? He looked at his watch and said it is 15 minutes to shekiya go borrow mine you still have some time))
So for example techeiles you say its impossible, ut at the very least there is a chance that its a mitzvah? 1%? 0.1%? There is no harm in having a blue string. However it isnt our mesora for whatever reason Hashem wanted it lost. so we really on the Rema (O.C. 9:5) that the minhag is to have davka white strings. Even if their is a chance (however remote) that we are missing out on an aseh.
If youd like a more controversial example. One of my Rabeeim in E”Y starts his Purim seuda shortly before shkiya on Purim (tes-vav since he lives in Yerushalyim). When asked why he said it is a beferish Remah 695:2 “shenohagin lehaschil samuch la’erev veikar haseudah hi leil tes vav” (Interestingly same Yeshiva as my other story above!)
however have a look at the 5 words preceding the above