I tried to search gotsugoushugi thingy, and apparently it mean plot convenience which would be similar to Deus Ex Machina (Or Diabolos Ex Machina for the story getting worse). Obviously the usage of this is always cause controversy no matter which side of the world, although whether the usage would destroy the enjoyment of the story or not is up to each of their own.
About Grisaia, let's just say I did think Meikyuu and especially Rakuen as the fandisc simply because there's no canon route in Kajitsu, something that Frontwing didn't provide at 2011 because they intend Kajitsu as their last VN and ready to scrap the trilogy plan. Of course, history speak otherwise and thus Frontwing continued the trilogy plan while recklessly expand the franchise (Phantom Trigger and Chronos Rebellion). Anyway forcing charage into linear VN obviously would cause issues especially with one of the ending is pretty much completed, so Frontwing tried to fix the situation by showing on how Yuuji help all five girls in the anime, which to say not really perfect but at least it get it's job done.
Speaking of VN series that turned into trilogy or duology, there's Galaxy Angel or Ryuusei World Actor which does the continuity better (Galaxy Angel ask us to chose the heroines at the beginning of the sequels, while Kinugasa (For all his ending writing fault) always wrote the sequel based on the available ending). There's also Sharin in which the fandisc is like Meikyuu, and Keiichi stated to Hozuki he chose Hinata as his lover. My point is all of those examples write the continuation based on the available endings, something that Frontwing didn't do with Grisaia, so yeah I can definitely see if you have the problem with it.
Anyway, for all of it's fault on connecting the sequels, at least the author manage to write nice character stories, which allow Kajitsu quickly to become well known VN with I remember did keep my eyes on by then active TL Wiki site to see the translation progress of Kajitsu. That's all for what I can write about Grisaia here.