It isn't too difficult to get there if you start with a "Heidi solo" within the house, and also pursue Sadie. The critical moment comes early in the VN - I think it was right after breakfast early on and you can choose to go see Heidi or go see Athena. If you go see Athena and tell her you want to pursue Heidi (not both) that's one way of getting there (maybe the best way); or go see Heidi. Afterwards, just stick to Heidi-centric compliments rather than both, etc. You don't entirely avoid Athena (ex., y'all go hang together at the bar so that you can get the Halloween party invite), but you just don't grab the solo Athena points - since you have the mod installed, the Sancho guide will be very helpful in minimizing Athena points, but without being an utter dick: don't do things like accept the ride home; no LAN party, no solo gaming with her, don't hang out with her after she hurts her ankle, etc. You don't make her a scorned lover and I don't think she moves out, but you never become more than just friends.