I think they will ship around 18.5 million and sell around 15 million

Last year my nephews did not know what is the NS, but this year, 2 have been ordered in my family for Dec 24th.

I have faith, I'll say 19,5/20 millions

Well, they had 1.79m on shelves at the end of last FY, if they did these numbers they'd have shipped 5.29m more than they'd sold to consumers! Agree with your shipments prediction but think consumer sales will be better.

17.5-18m sell-through, 18.5m shipped is my guess.

20 million shipping units is not the same as selling trough to customers. If Nintendo is fine with having more units in stock they could still easily outship the PS4 this year, even if they don't outsell it. 

Is there any hysteria in Japan about the very recent release of Pokemon ? These things could be strong sign to predict how will be the Q3.
It seems that no. Instead we see negative new like this one :

https://gonintendo.com/stories/322918-amazon-japan-disables-pokemon-let-s-go-pikachu-eevee-review-scor

It was disable because of last year games backlash, and not wanting a repeat if fans didn't like these titles. 

17-18 million.

If they are so close, you can bet they will overship 500k units more in order to hit their goal.

Isn't illegal to do that ?