I refuse to be drawn into any kind of debate about opal, opalescent or opaline glass - I still have memories of our endless toing and froing the last time we went down that road
I'd suggest that determining opaline in a turquoise shade - on the screen - is a very big ask ... it isn't simply turquoise coloured glass. As you will know better than me, opaline was a speciality of Baccarat (first half C19) though it was also big in the Gulf of Venice in the C17 - but certainly turquoise was one of the colours made in opaline, and the French were renowned for making the stuff. Usually in pastel hues, and some degree of translucence is essential which is then opacified by the inclusion of ashes of calcined bones and then metalic oxides for colouring. Unfortunately, so many folk seem to think that almost every piece of opaque coloured glass is opaline which does make for confusion.
Regret I can't comment on the veracity of museum exhibits - I have enough problems researching my charity shops finds, but take your word for it that they are knee deep in errors - I suppose we tend automatically to assume that as experts they're going to be 99% correct.
Coming back to John's top hat, and in comparison with your links, it does seem, as you've said, that turquoise was a colour used for trailing albeit rarely, though turquoise used for a solid trail for edge trimming - as on John's hat - seems non-existent, and it was in that sense I was a tad troubled. On the face of it John's hat is unique, at least until another pops up - so I'm prepared to go to a tenner John it that helps.
- but I'd suggest a very important find and well done.