It's possible that date was picked because it's closer to WW release date (whenever they announce it for the west).

Right, but the producer/developer wouldn't play his game for the last time knowing that there is still 4 months of testing to do.

That's what I think.

Why not, play through the game as standard then let the testers try and find any bugs and issues that may occur, it's like fighting games going to arcade first to be tested for balance for months prior toany console release.

It's not about how long it takes. It is about when every single retailer has the right amount of store-space for the game (which costs money.) It is about aligning advertisements with the release date so the maximum amount of coverage and hype is built before it is released. It is about choosing a good launch period in which no other releases conflict or there are no extended droughts. There are so many economic factors to consider which are greater than software development when selling a game. Otherwise developers wouldn't need publishers as much as they do. 

Isn't the real answer Golden Week since the game is available for pre-load now. Securing retailers only takes a month or so.

Yea, most Nintendo games are kept months after their finish just for polish which can be a tad bit annoying but I guess that's the right mentality.

It's literally nothing like that at all. He said it was his last playthrough. No director is going to have their last playing of a game to be of an unfinished version of it.

I'm more concerned as why to why it's not released globally everywhere at the same time.

Again, I don't think that's the case here. Not that there was no long period of polish, but that that period was likely months ago. The game was slated for last year. I have a feeling they used the months at the end of last year for all that polish, and it finished late December/early January.

I seriously think that there must be another reason. By looking at how complete the game appears to be, there's no reason it shouldn't be launching at like early to mid March. Like others have said, I think it might have something to do with the localization.