Ok so let me understand, this pool has the options for 1-5 million, and 5-10 million, but not a 25 million option?

Are you real?

20-25 million by the way.

I was thinking of adding a 20-25 million option, but I wanted to have the mid-point of the scale to be 'momentum being generally the same' with two other options away from the mid-point on each side. 

According to Nowinstock, Japan has pretty much been out of Switch's since around January 12th. I am curious though whether a new shipment would be potentially damaged (it will not move off of shelves quickly due to decreasing demand) because of the release of MonHun on PS4; afterall PS4 appears to be out of stock to based on the information from NowinStock so there has been a major uptick in demand there (potentially at the cost of Switch?). Am I overestimating the negative impact the exclusivity of that game on a single home console will have on Switch's momentum and hardware sales?

I think you are being a bit pessimistic tbh. If someone wants a switch, they will likely wait until they can procure one. Of course there may be some impulse buys of MH which may affect Switch sales but only temporarily. Am sure things will return to "normal"  in 1 or 2 weeks. There has been much ado in the past on the media create threads about whether Switch was cannibalising PS4 sales. Nearly all of the PS fans insisted it wasn't. Therefore it works both ways. The consensus was that they are not competing directly, or there is, at least only a small overlap.

I see. NPD's Mat Piscatella, stated something to the same effect about the audiences not overlapping in a major way and Switch largely addressing a different segment of the market (at least in the US; and he should know since he access to all market figures). All of this is really making me believe that the video game industry as a whole is much bigger than I actually believed it was.

I would said around 20m.

Well i guess it is, if you factor in lapsed gamers, nostalgic gamers, the huge casual market that comes and goes, regular gamers, families and hardcore gamers there are lots of different demographics. Nintendo and Sony were originally after a similar demographic, so there is some overlap, although it seems quite a few people own both consoles. With LABO though, Nintendo is spreading out its reach to include other demographics, so hopefully will be able to diversify and massively extend their userbase. When Animal Crossing comes out, Nintendo will be reaching out to one of the rarer demographics. Female gamers! (Hope i'm not deemed as being too sexist here!)

Monster Hunte: yes it will have an impact, but as you noted, it's not portable on PS4, which will both reduce impact to Nin and reduce sales of mhw in my opinion.
Price: I'm amazed Nintendo got away with the orice but now that it's well established, it is not a big deal. The market understands the price and Nintendo can slash it any time if really necessary.
Software: this is the big one. As you implied, the definite known lineup is nothing special, ports and second stringers. If Pokemon gets out this year, great. If Metroid is out this year AND if it's really great (botw great), it can have an effect too, thou not as much as Pokemon or Zelda or Mario. Hopefully Nintendo has something amazing waiting in the wings, because neither game is a lock for 2018 yet and if Metroid is average or "just good", it will not be enough.

I'm not expecting Metroid Prime 4 this year but even if it comes, even if it's great, it won't move the needle. In the Nintendo hierarchy (as you rightly imply) Pokemon & Mario are king, with Zelda historically selling far fewer copies worldwide than those and Metroid barely in the race. Sure, all the Nintendo fans here will be buying it, but I'd think that it will sell only Splatoon 2 numbers at best and I don't think anyone buys a Switch for Metroid. It’s also worth bearing in mind that Metroid Prime's strength in the past was that it delivered a shooter on (post-N64) Nintendo systems, and we've already had Doom with Wolfenstein 2 incoming. I don't think a first-person single player game (albeit a system exclusive) is as important now as it was in the past.

I had read the title as "Swiss" for the first time and was scratching my head thinking who would think Swiss sales are relevant to make predictions.