To be more accurate, media products are event-driven and seasonal. Their distribution method is not like our iPhone (or smartphone in general), in which we have a pretty stable demand throughout the year for the phone. It's more like music album sales back in the days. So yeah, the first few weeks after the event it's tied to, would typically determine its lifetime sales.You mean like how endgame and no way home tickets sales show that they were/are gonna be the highest grossing films of the year they came out?
It's mostly arguments and debates against the fans that are pro ss & sakura, against the anti SS, anti sakura or both. With the arguing whether or not the actual novel has sold well or not in Japan.
With both sides using sources and input from fans from Japan about how well the series is doing. One example is here claiming a Japanese fan told this person that apprantly the sasuke novel is on its 5th reprint, showing that its selling well.
https://twitter.com/...ZjbTyqSzSQ&s=19
I personally would stay away from debate that's not referring to official figures. It's not worth your time. Stay away from folks that have been consumed by confirmation bias. Nothing will change their mind and you'll better spend your time elsewhere. If the light novel sold well, it would have been in the Oricon chart weekly, half yearly or yearly. It's that simple (as far as Oricon tracking goes).
The Twitter account owner (WSJ Oricon?) was sharing information on a good intention, but he/she is still not an official source. Not to undermine him/her or anything, but I'd prefer to quote from the official website like Oricon (there are legal consequences if Oricon spread misleading information). If he/she has the official information, share it with public alongside his/her quote. It's good for everyone.
Reprint is basically a mechanism for which the producers back-order to the printing companies to add or to replenish supply. It's a common practice in the printing industry. You'd order a high volume to get better discount from the printing companies if you are confident that your product will be well received (based on market research and other go-to-market strategy). You'd order lower volume if you'd like to be more conservative, observe how market react to your products and reprint as needed. It's all about maximizing your gain/profit.
Remember that books in circulation does not equal to the sales figure. So, it's a pretty wild stretch to correlate the number of reprint as a measure of product success. You back-order (reprint) 10k copies 5 times does not mean your product is more successful than a product that's sold 80k for the same time period.
When it comes to light novel, my reference for sales trend was "your name". It's seasonal and event driven. By September 2016, the light novel had sold around 1,029,000 copies. In the second week on September 2016, the novel had sold around 112,000 more copies. After the wide release of the film adaptation in August 2016, the novel reached the top place in Oricon's weekly bunkobon sales charts for three consecutive weeks. The novel has sold over 1.3 million copies as of December 2016. They never bragged about how many reprints they did. They're on Oricon chart that year. If your product is good, it'll be well received. As simple as that.