I’m a little confused by this conversation.
Dy perhaps you (or anybody else) can help.
It seems to me that by definition any minhag practiced by frum Jews has some rabbinic backing, including many brought above (banging by haman, not washing for shalsoh seudos not sleeping in sukkah etc) If there is no “justification” then a forced one is brought or a daas yachid is relied upon.
What is an example of a minhag with NO halachic support about which the expression “minhag trumps halacha” or “minhag brecht a din” would be (wrongly) applied.
DY the reason I singled you out is this line “where the rav has told people to abandon minhagim which are clearly against halachah,” do you (or anybody else) mind listing these minhagim?