The main game would still feature our one and only MC. Least to say, the non-main character we see the POV of in this interlude would only be featured exclusively in this interlude. Other characters, with the exception of Heather, would also be exclusive to this interlude only.
Well, I would be very, very careful with how you handle characters from the main story that appear in the interlude or vice versa. One wrong move there could cost a lot.
For me personally, I dislike multiple MC games. They make the game far less immersive and take away from the characters I care more about to characters that I care nothing for.
Anyway, one bug I noticed when doing the walkthrough is that the
the sex scene variable is never set. There is a dialogue split for it, but the variable itself it not set to true/false for that choice.
One other thing: I would assume you have this planned already, but since the choices in the Interlude will affect parts of the main story, you will need to somehow bring the choices we made there to our main save. I am not sure how that will done since I do not know Renpy that well, but I would assume that is something you have looked into.