Hey bts. I'm going to my posting next month.
I did have the final interview with the country office. I don't know if I was the only candidate they interviewed, but the interview itself was pretty technical (mine was, to be honest, a lot more technical and harder than the NETI interview). I guess it can vary depending on the country office, though.
If I were you I'd just go on with the job offer you accepted, the unicef process can take (yet) more time. If a firm unicef offer does materialize, I'm sure your employer will understand and unicef will let you give fair notice.