Some helpful info with InterfaceOptionsFrame_OpenToCategory
This is used to display a specific addon's settings in Menu -> Interface -> Addons...
Call this function twice (in a row)... There is a bug in Blizzard's code which makes the first call (after login or /reload) fail. It opens interface options but not on the addon's interface options; instead it opens the default interface options. If you call it twice in a row, it works as intended.
Yep. It's been like this for years. I don't recall if this is still the case, but I think that for a time it was only this way if your addon was out of view in the left scroll frame (the list of addon pages in Interface Options).
"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff, The Tao of Pooh