Welcome to FT, chevievie!

I think the part of your question about how to spend your time in your various destinations is the hardest to answer, as it's all about personal tastes. I would only encourage you to do some research and thinking about what you'd like to see and accomplish in those places. There's certainly enough going on in your cities/countries that you could indeed spend months in any one of those areas. I'd pay attention to local weather and how it might impact your timing - rain/dry seasons in Fiji, hot summers/cold winters in the US, etc.

As for the airfare, I'd ask a good travel agent, or contact Finnair or American Airlines and ask about the Oneworld "Global Explorer" RTW ticket, which would allow you to visit Fiji (while the more popular "Oneworld Explorer" wouldn't.) A 26,000 mile Global Explorer bought and started in Finland would cost €2110 plus applicable taxes and fees, and would permit a route like this. A Global Explorer allowing 29,000 miles of flying, and more stopovers, would cost around €400 more, well under your previous quote.