I found a book published in 1886, which contained an interesting statement pertaining to the realization of the 合拗音:
Natives of Tōkiō and the Northern and Eastern provinces omit the w after k and g in such words as Kwan-non and Hon-gwan-ji.
This seems to state that Modern Japanese speakers outside of Tokyo and the "Northern and Eastern provinces" still regularly pronounced the 合拗音 as recent as 1886. Wikipedia seems to state that the disappearance of the 合拗音 started in the Edo period. The way it's written here seems to give the impression that the "proper" way to pronounce was including the 合拗音 and that the natives of Tokyo and the Northern and Eastern provinces were the ones who were different, rather than the ones continuing to pronounce the 合拗音 (although this may just have to do with the 歴史的仮名遣い spellings which explicitly include the ゎ's). That got me wondering, when did the 合拗音 end up dying out, where specifically was it still preserved, and why was it still preserved in these areas after it died out in the rest of the country? What was the reason for the 合拗音 dying out? And are there any dialects today, however minor, that still keep the 合拗音? Thanks in advance.