Hi JeffB.
When I say the 'normal tune' I mean the one R A Smith put to Tannahill's words in ThE Scotish Minstrel 1821. This melody was printed continually throughout the 18/19th century and was first recorded by Alma Gluck in 1914 and is printed in Norman Buchans 101 Scottish Songs 1962.
Tannahill's set his lyrics to The Three Carls and the first printing of his lyrics to this tune was in 1810 as I posted.
These two melodies are different and Smith eventually printed the Three Carles tune in 1824.
The 1817 tune referred to as The Braes o Balquhidder had to be the three Carles tune as Smith's setting did not appear until 1921.
Brochan Buirn and the The Three Carles is the same melody and so is St Helena when it appears.
James Watts composition is a Bonapart song and the lyrics have nothing to do with The Braes except he says his lyrics should be sung to the 'current' tune associated with the Braes, which, as I pointed out has to be the Three Carles/Brochan Buirn which Anne Gilchrist said her mother sang it to. By the way, Watts was from Paisley as was Tannahill.
It makes sense then that the tune called St Helena is in fact The Three Carles and this can be heard quite clearly.
Because Tannahill's and McWilliam's lyrics were conflated, the tunes must also have been mixed together by the travelling people and again this can be heard in all the Ballymena versions in the Greig/Duncan collection.

To summarise: Mary Black sings the Bonapart song to the Three Carles Tune which Watt called The Braes o Balqhuidder and this is the tune McWilliams called St Helena.
The tune now sung for the Braes can be found in publications after Smith's first printing in 1821.
I call this the 'normal tune' and you can hear Kenneth McKellar sing it on Spotify.
Don't confuse the melody of the McPeaks song as it is entirely different, only the lyrics have been 'absorbed'.
I hope this is clearer but if not ask me.