I....what? The WiiU sat on shelves unwanted and unloved for years and eventually retailers just moved it out to clear shelve space. We even had threads on this site about the crazy (unofficial) discounts it was getting even though it had only released six months ago. There were provably no artificial shortages going on with the WiiU. It didn't sell and by the time it picked up a litte Nintendo had stopped production and the Switch was in the works, so rather than kick up production again wich would have been costly and ineffective, they let the leftover supply 'carry' it until it's demise last year.
Artifical demand only 'works' if you're actually trying to sell units. That's why it can't apply to either NES mini or the WiiU. Nintendo dicontinued both of them, thus they didn't gain anything by 'creating artificial supply'.
Amiibo were a surprise hit and sold out because they were popular. Nintendo did up production and most of the original 50 are readily available now. I get the feeling they downsized manifacturing quatities again for the newer Amiibos, with waning Amiibo popularity though. The Zelda BotW ones haven't seen a single restock since launch for example.
I think you hit it right in the mark about amiibos. As popular as Nintendo characters and their respective IPs are, it was probably hard to gauge their potential popularity upon release with amiibos, especially during a time when the 3DS and the Wii U were selling below the rates of their respective predecessor's. Wihle toys-to-life was still popular at the time, it's not too long ago they faced a decline. I don't know any numbers right now, but I don't think the sales are as big as they used to be 3-5 years ago.
As for the NES Classic Mini, it probably was inexcusable sell low productions. Due to the nostalgia, there's still a huge craze for retro gaming and the NES Classic was a big deal, especially at the $60 price tag it had. There's a possibility it discontinued due to having similar chips and parts to the Switch so Nintendo had to put more focus on the Switch, obviously, since its the new console and part of their bigger investments while the NES Classic was meant to be more of a smaller but still popular thing.