Heather seems to really love Tommy. In fact, she uses Fuckface at the party driven by pure spite. At that point she sees us only as a means to an end: to spite Tommy.
Heather loves Tommy without any doubt. But she is dissatisfied and unhappy in her relationship with Tommy. And here we have the dilemma. You're right about Heather wanting to use the MC, she says so.
I always make the decision for Heather to get in the MC's pants, which however keeps Heather from moving on. The MC can prove two things with this. First, he stays true to his chosen LI and second, he is a DIK brother in that he does not fuck Tommy's Girlfriend. If he does, living with Tommy will really suck.
A breakup of Heather and Tommy will happen either way, because even if the MC doesn't do anything with Heather, Heather will realize so the relationship with Tommy has no future. Heather probably only agreed to this form of relationship because Tommy wasn't sure. Actually just like the MC in the hangout conversation with Josy and Maya in EP5. Maya actually gave the MC a free pass to try it out. Heather will have done the same with Tommy, but Tommy didn't grow up. Even Quinn notices that.
But by rejecting the MC, Heather realizes that it has nothing to do with trying things out or anything like that, but with a person's attitude. Tommy doesn't want to change and Heather realizes that at the Halloween party.
I am assuming so Heather will get involved in the fight between Josy, Maya and Tommy in early EP10. She is still in the kitchen. She will side with Josy, Maya and the MC and leave Tommy.