Thank you for such a well written view on the engine itself. I appreciate this a lot as it helps understand things as a whole. I don't know why SS has as many issues as it does and I do hope they figure out why it's so troublesome.
Does Ren'py have multiple versions? Is it possible that they're using an older, more archaic version than newer ones with newer features? Like Ren'py V1 that's very basic while the newest is Ren'py V7 with a lot of built in features for example?
It has multiple versions but the few changes that are made are not that significant.
See, Renpy is basically an accessible language for people with no programming experience to be able to create Visual Novels. The simplicity comes at a price: when you want to expand the scope of your game, you have to start writing more and more redundant code because the language does not give you easy solutions to complex things (that's not the intention).
But Renpy is based on Python and you can use this language to create more complex functions that will help your code become more optimized and practical to use in complex situations. I believe that is the case of the Summertime Saga. The problem, in my view, is that some programmer made a mistake early in the process and the game began to be built on a base that is not that good. This results in hundreds of bugs per update and players being unable to use basic functions such as loading old games, rollback text, etc.
But you see, I'm guessing this, because I don't read the STS code. I formed this opinion based on playing one version of the game (0.17 I think), not analyzing the code. Anyway, in my opinion, one of the strangest things in the Summertime Saga is that, except for the minigames, there is really nothing in the game that would justify a very intense modification of Renpy. Much of the gameplay can be easily recreated using the basic functions of Renpy without the programmer having to spend much time. It's not as if an intense optimization was crucial to creating a game with the same style as the Summertime Saga in Renpy. That's why I think someone tried at some point to reinvent the wheel and ended up doing a monumental shit.