fearoffish

Final update, instead of just unticking the checkbox to disable the input source switcher...put a new keybind on them too. Something you'll never use like ⌘-⌃-⌥-F6 Doing this fixes that final and annoying "Why the hell has my input source switched again?" problem. Phew!

One more side note, I think there's a bug where the shortcut still works for Input Sources in the background. So when you add that second Input Source, try and add a variant of what you already have. e.g. I added English - Extended ABC (or something like that), so if it switched it didn't matter. Before I did that, I was being switched to Coleman when I used ⌃-␣

In case nobody has found the solution yet, it's the language input switcher. If you go to System Preferences -> Keyboard -> Shortcuts, in the list on the left there should be "Input Sources". The shortcut you need to untick is in there. If the "Input Sources" isn't shown on the left, then go to System Preferences -> Keyboard -> Input Sources (tab at the top) and add one, any will do. This will make the shortcut appear in the list now.