I like
Catriona. Not that pronunciation so much, because I'm it's just the pronunciation used by people who've read it in a book and not known how to say it. (Like pronouncing
Aisling Ace-ling.) Still, I have met one or two Scots who use 4 syllables. It probably is prettier that way.
I think the
Riona part seems contrived. Shortening a 4 syllable name to 3 just wouldn't come naturally to people.
Riona would be a fine name on its own if you like it.