If it wasn't for all of the supply problems, I might have agreed with you. As it is, it's too difficult to reach the million that soon, not enough consoles around.

Most of it depends on how many Switch consoles are out there in week one but it'll come close specifically because most of the people who bought the Switch early in Japan probably did so with Splatoon 2 in mind and even those that didn't will probably buy it to be a part of all all the excitement.

Okay.  Nevertheless, there is no way it will sell 1mm on Japan in the first week.  It will not have a 90% attach rate within one week.  

"A possibility of Splatoon 2 hitting 1mil First Week in Japan including Digital?"

If it has like a 90% attachrate.... suuure.... but probably not gonna happend.

85% Attach rate! First Week
up to 95% week 2

from then on will be 0.85 to 1 console sold! for a few months.