So, the general reason that the walkthrough doesn't tell you things like what the fastest way to get on any particular route is, is mostly that it's really just designed to tell you what choices do and let the player then decide what they want to do based on that. I can't tell players what the best way to do things is, because there are usually multiple ways to achieve the same outcome. If a player wants X character on slave route and there are multiple ways to do it, the best I can do is tell them when a choice will specifically lock them out of that, or when a choice will directly lead to that. I can't tell them what the best way is because that will depend on what that player wants beyond just "X character on slave route". It's intended to provide information, rather than recommendation. Unfortunately due to the relatively complicated nature of the game, any guides to achieve a specific end goal would likely be better placed in pdf format.
Currently the fastest way to do anything with Cassie is to flirt with her at the start of the game. That gets you onto her romance route, which is her only currently available route (Though it's more of a fwb kinda arrangement at this stage). You get a second chance to do this shortly after becoming the Magistrate as well, since the very first opportunity is sort of like a hidden possibility. (It's not very hidden, it just requires you to make two choices that aren't immediately obvious, which is why you get the chance to get on that route later as well.)
She works a bit differently to the other girls since she's kinda down for anything. When her final route selection comes up, it will likely be just a straight up "Choose a route" kinda thing rather than the system in place for most of the other girls. This is part of why she's fwb instead of an actual relationship. Makes it possible for her to transition to slave without me having to write heartbreak.
Issue you have with the prompt about the threesome is that that threesome can only happen if you're on Cassie's romance route, but the prompt you saw occurs on Riley's sub and slave routes. It's there because it's possible to mix the routes, but if you only did full romance and full slave, then you won't have been given that threesome option when you saw the prompt.
Annoyingly, I can't have the walkthrough react to what the player has or hasn't done just due to how it works, so the intention is that any player who hasn't been given the option regarding the threesome will just think "Ah, that doesn't apply to me" rather than "Why didn't I get that option?". If I find a way to make the walkthrough react to the player's previous choices then this should become a non-issue, but at the moment I have no way of doing that unfortunately. (This possibility would also mean some of the more wordy walkthrough entries could be cut down to just the stuff relevant to the player's situation, resulting in me being able to increase the font size for some of them. It's on my list of things to eventually implement, I just don't have a way to do it right now.)
