I've never heard it pronounced Ro-enn-ah - always Row-een-a. I think if you choose an alternative pronunciation you are always going to have to explain to people how to say it so you are better choosing an alternative name or spelling it differently otherwise people will automoatically say Ro-ween-a.
Saying that, I don't like the name, I think it's too dated and not that attractive. I do like
Rowan though, but only for a boy.