And why should they release both handheld and home console at the same time? 

They release it with one year gap in between. 3DS successor 2016 and Wii U successor 2017.

Because, if the NX is the fusion concept in the end, both the console and the handheld will be inexorably connected.So if thats the case, they will need to launch togheter, or at the very least few months apart.But we will need to wait for E3 to see that.

They need to get people buying both, the handheld and the home console. If you launch both at the same time people will ignore one. If you bundle both, it will be too expensive. So they need to launch both at different times. Ideally at the end of the year because sales will be the strongest then. So one November 2016 and one November 2017. An no, it won't be a fully fusion system. Iwata has already denied that long ago. Of course they will work closer together then ever before, but the won't be replacable by each other. There will still be exclusive home console games and exclusive handheld games. Even the unified system has the reason to avoid software gaps after launch because Nintendo can easily port games to the other system. That was all said by Iwata.

Going by precedent, Nintendo typically has a direct at the end of the year/after the holidays. Xenoblade, Hyrule Warriors, Fire Emblem fates were announced in these directs

Basically, whole point of fusion and unified platform would be lost if they release them with one year gap, in worst case they need to release them with only few months gap, but ideal and most effective for whole idea is to release them in same time.

I think there are good chances, because Nintendo very easy, fast and without many resources can make Pikmin 4 for Wii U using Pikmin 3 engine and assets, same like they done Pikmin 2 after Pikmin 1 on GameCube.

Yeah, we need release dates for FE: Fates, SMTxFE and Bravely Second.

Thats why the Fusion concept is a concept.Its a theory of something we dont know.Thats just one possibility of what the NX could be.It could surely be just a "regular"  launch.We dont know that yet.Though all this talk about My Nintendo and the cloud save data indicates that the fusion concept might be right, or at least that the console and handheld will be more connected than ever

They dont need people to buy them right away, they will be fine if just a people buy one device first, but in that case both consoles will be very attractive because unified platform, hevi integration and cross play...out of gate, instead, "here is one console of unified platform but if you want to enjoy all benefits of this platform you will need to wait next year for another device"!?

Whole point of fusion or unified platform would be lost if they don't release them in same time. Developers need to develop same game out of gate for both consoles, they cant develop same game for two console if they have gap one year between them.

Of Course that unified system will avoid software gaps and droughts, thats whole point of fusion or unified platform, developers will develop one game in same time for both devices, and not like before two completely different games for handhelds and home console even if it is same game. There will be some exclusives for NX home console, but big majority of games will be playable on both devices.

I think that they would prioritize getting Sega Genesis on the eShop before Master System