After going back to the January Famitsu issue, I'm more convinced that this case does take place between DGS1-4 and DGS1-5. The blurb there talked about how a letter from Susato to Ryuunosuke "brought to mind another case, hidden until now".

When I first saw that, I assumed it was referring to something like an Asougi flashback case (which was my guess for what would start the game for a long time). But as we all know that's not what happened. So instead it sounds like Susato's letter - which I'm guessing is recapping the events of DGS2-1 to Ryuunosuke - reminds Ryuunosuke and Holmes of this case, presumably because Souseki was involved in DGS2-1 as well.

What I still can't wrap my head around is why they would structure it like this. Unless this case is much more deeply linked to the overall DGS story arcs than we think, flashing back to this period adds nothing from a plot perspective or a character perspective. My only real guess is that Susato won't get back to London until case 4 or 5, and they wanted to have another case with Ryuunosuke/Susato/Holmes/Iris all together before that. I guess it could also serve as a way to get new players up to speed, though they sure seem to be marketing DGS2 as "please play DGS1 first", lol.

or it has connections with the Layton Brothers Mystery Room.

I really want something like this. That game really needs a sequel...