I’ve expressed before my feeling that the decision to reveal Kagura is dead now in the Inroi Route added to Shiny Days in 2012 was a retcon. When Kagura is name dropped in the Summer Days routes it seems like a reference to someone still alive.
My attempts to decipher the Timeline of the Overflow Universe lead me to placing Summer Radish Vacation 2 as no longer a Prequel/Period Piece but in fact set basically when it was released, the Summer of 2004. Which would also make it either the year before or the same year as the events of School Days and in turn Summer Days.
Summer Radish Vacation 2 and the Days Series are both presumed to follow the same ending of Summer Radish Vacation, where Shun, Kagura, Youko and Mai form a Polycule but Rina and Rio are both taken by Tomaru.
In Summer Radish Vacation 2 none of the main protagonists of the prior game are on screen characters, but according to the Wiki it is referenced that Kagura is still in a relationship with Shun and that Mai had left home, no reference to Youko seems to exist.
That reference to Kagura is Summer Days can be interpreted as being consistent with her still being with Shun, the reference comes from Shun, someone playing this not knowing the full genealogy might well assume Shun was talking about his Mom not Mai’s. And of course the entire premise of the Days series presumes Mai and Youko left the Polycule for some reason (what Setsuna assumes about why her Mom left her Dad may not be accurate given everything else she probably doesn’t know).
In my PureMail post I talked about the awkwardness of most people, especially in the West, referring to these prior Overflow games as if they’re prequels to School Days when the relationship was originally the opposite. It’s easy for people to assume Youko and Mai are the Waifus who matter most in Summer Radish Vacation because it’s their involvement with Shun that created two of the most important Waifus of School Days/Summer Days. But it was made first and foremost as a sequel to Snow Radish Vacation. So it was in a sense Kagura who was originally thought of as the female lead, giving her a true Happy Ending after the sham of her supposed happy ending in Snow Radish Vacation fell apart is arguably the most important objective of the game. Maybe she’s even the only one ever actually properly married to Shun (that they have the same father is probably not legally acknowledged, I'm not even sure if they know about it).
I view Kagura as the true most important protagonist of this meta franchise. Her getting to finally live a happy life after the events of Summer Radish Vacation is kind of important to me.
But all that is assuming they all follow the same ending.
I kind of wish more of these multiple ending games would have separate sequels for separate endings. What if we could pretend Summer Radish Vacation 2 and the Days Series are exactly that?
Originally Overflow released an “Adultery Chart” that said Tomaru not Shun was the father of Sekai and Setsuna. This is clearly firmly retconned by Summer Days but maybe School Days was first written with that idea in mind?
In Summer Days it is contrary to what I first thought implied that Shun being Sekai’s father is also known to both Sekai and Setsuna, it’s just not explored as much because Sekai doesn’t interact with Shun.
I already laid out what definitely is pretty consistent between Summer Radish Vacation 2 and Summer Days.
If they follow different endings, perhaps it is the fate of Rina and Rio that’s different? School Days does refer to there being an Inou who was about Makoto’s age in Grade School, which the Wiki presumes is probably Rio’s daughter Risa Inou who wouldn't exist if Tomaru didn’t take Rio since Shun as scummy as he cna be sometimes I don't think would impregnate Rio at that young, in fact he certainly doesn't have the means Tomaru has to make it biologically possible. But what if this Inou could be a daughter of Shun and Kagura? But if I'm correct on them somehow getting legally married their kids would probably be named Hazama.
If there is actually a problem for Summer Radish Vacation 2 and Summer Days being in the same timeline it’s Itaru Itou. Itaru is supposed to be usually staying with Tomaru and SRV2 depicts Tomaru’s household seemingly during this same time period or only a year prior. Maybe she’s absent because she spends a chunk of each Summer staying with Makoto and their Mom as seen in Summer Days, but not all of it. Maybe Tomaru is only so interested in keeping Itaru around because he doesn’t have any Inous in this timeline?
If they are separate timelines then perhaps Rideru Takatsukasa and Makoto Itou are in some cosmic sense different versions of the same entity? Both are born after the events of Summer Radish Vacation, both are children of Tomaru by a Nurse, both are protagonists who start their debut games knowing who they want to romantically pursue but can still wind up with someone else. But for Makoto it’s the first choice who turns out to be a Rapist. And Rideru does seem like a much less passive character then Makoto.
But they probably are the same timeline and some explanation for Itaru’s absence from SRV2 can be dreamed up. It could be the Itou family only wound up in this shared custody situation less then a year before the Summer of Summer Days, I don't recall if the games themselves mentioned a timescale for it.

