The game from the beginning had "5 routes" as you put it, since it always had the content for each of the main LIs, plus the side pieces. And from the very start all of it was optional, meaning the branching code was there, too. That parts of this content aren't accessible to every player in more recent episodes doesn't change anything in regard to the workload involved. At least on the developer's side.there is one big problem, and its the Branching, game was inicially split in 2 playthrough (DIK/CHICK) , then went to 3 now we are at 5 playthrough, making branching is good but its to much demanding and time consuming, making the episode feel short, for me to unlock full gallery i had to do the same things 5 times, only to get like a bit of new content.
As for the gallery unlock, just use a script which will do it, if that's all you care about.