“but it is a machria where there is a machlokes,”
Are you aware of any machlokes as to wther we dont do mitzvos due to lack of simcha?
“He very possibly would not have, but the point is that someone of authority did come to that conclusion,”
Great! that is my point. you sort of admit that what drove the Rema to say that we dont duchan because of lack of simcha. Is not necessarily that after going through hilchos nesias kapayim or hiclhos simcha he came to this pesak. Rther after noting our EXISTING practice he gave a reason for it( that he may very well have heard from his Rebbi)
Fair?
“If no justification were found, we might assume that the change was made without halachically valid grounds,”
Agreed. Though where we differ is, that I say by definition a minhag practiced by shomrei torah umitzvos has justification so if “no justification were found” it means we arent looking hard enough (even if it is as weak as not duchaning because its too cold for kohanim to go to mikva or a hekish of gedilim taseh lecha to ki yikach ish).
I believe this is our point of contention. however earlier you couldnt identify a minhag practiced by yirei Shomayim that couldnt be justified (with the possible exception of eating indoors on Shemnini Atzeres)