Nintendo have stated that they have upped their productions levels from 1m to 1.16m per month, stating that they will be able to ship 14m units until the end of March 2008.
Yes, potentially if all those units get sold, they will have sold 20m units in total by March 2008.
You state that you think the Wii will have sold 18m units by the end of this year....this is not feasible going by Nintendo's stats, that they will have only managed to ship aroud 16.5m units maximum by the end of 2007.
Other factors that you fail to think about is that demand starts to slow down...although there are no signs that this will happen...we have 8 months until the end of the year and surely the odds are on that demand will slow down to some degree, especially seeing as there are effectively 3 next gen consoles on the market now worldwide and prices of both the 360 and PS3 are likely to come down at some point this year.
Anyway, having said all that, I still reckon that Wii will be top of the next gens by the end of this year at 16m units with the 360 at 15m and PS3 at 10m.
Prediction (June 12th 2017)
Permanent pricedrop for both PS4 Slim and PS4 Pro in October.
PS4 Slim $249 (October 2017)
PS4 Pro $349 (October 2017)
