I've always pronounced the word "Southern" to rhyme with howthurn. I know most people say it like "suthurn" instead. I didn't realize that the way I pronounce it is considered weird until recently!
Unfortunately our linguistic history is a huge tangle and there are few safe assumptions. Depending on where you are in Scotland, the places names might derive from Gaelic, Pictish, Welsh, Norse, or English, and then they probably got Anglicised at some point but it could have happened at basically time within the last five centuries. A substantial number of the non-Gaelic ones are doubly messed up because they got Gaelicised first and then the Gaelicisation got Anglicised. Glenisla is a good example - glen derives from Gaelic, and nobody is sure where isla comes from.
Still, Glenisla is a lovely area! Lots of good hikes there. I hope you had a good time.