To summarize your questions, we have to consider that Moonripple Lake, overall, is meant to be an NTS game (and i open again the Pandora's Box on the NTR/NTS genres). Therefore the existance of an official relationship between MC and MLI and the presence of an MLI to begin with is a key factor to adhere to the genre.
In general there can't be swinging or cheating if there isn't a somewhat commited relationship and the girl is just one of many possible fuckbuddies.
About what's special about Suzie... well, nothing really. Of course she is a well written character, that came out with a high level of accomplishment and she's very popular among fans of the game, as well as Cee, but the reason why there is so much attention on her is the NTS (or possible NTR as some wishes for).
Now, not being a fan of those genres, especially NTR, albeit i enjoy the occasional NTS/sharing scenario in games, i can't be sure i understand the appeal of the whole cheating based story.
Once a guy here explained to me that the cool part of NTR is to see the female character in increasely extreme and degrading situations, and i though it was a good explaination at the moment. But then you would enjoy any female corruption game just the same if that was the case, and it looks like it isn't, for many. IMHO, and i beg pardon from all NTR fans if i'm wrong, i think putting themselves in the shoes of the cheated man and to see the situation unravel by that point of view is instrumental to the kink.
So, it has to be Suzie who gets drilled hard by strangers. Was it Drago's mom or the shopgirl, very few from the NTR/NTS crew would really care, i think.
Is it a matter of that they do not really know what they want fully and are leaving their options open? Or are they leaving open options to pivot if they sense that such where the money/interest is?
The devs have to sell the product, at some point. They may start with an idea, but then they see the reactions on forums like this one or from the Patreon, they start to listen to the feedbacks, albeit they might end listening to the vocal minority, they start to poll to see what the players prefernces are and they might star to change the take on the story.
Some game appened to go rather astray this way.