Im not sure if we are really arguing
You cited athe Magen Avraham earlier that discuses the parameters he of “minhag Oker halacha” he does not say there is no such thing.
As far as Duchaning. Do you understand the Remah, or any other Achronim that there is something halachicly wrong with duchaning? IS there even a daas yachid that holds birchas kohanim (more than any other mitzvah?) requires as n absolute prerequisite to be done besimcha and that according to those shitos, sefardim are making a beracha levatala?
Does anybody hold regarding birchas kohanim, halachos are:
1) need to be a kohein
2) need a minyan
3) needs to be be simcha.
Isnt it obvious that these reasons given are justifying an existing practice? (as to how the practice came about maybe it is one of those reasons, maybe another, that isnt really my point)
My point is there is no deah that actually requires simcha for birchas kohanim, in other words there is no deach that holds HALACHICLY we shouldnt duchan today. nonethless we dont duchan because that is the minhag