I understand what you're saying also, but if I phrase it like this, the important relationship dynamic for the plot currently is that Leia is antagonistic to William. Katie is going between flirty and antagonistic, and this isn't really central to the plot, although it will affect it. When Katie forgets to be angry, she tries pushing William's buttons just for the hell of it, plenty of teasing both ways. With Leia, William is on guard trying to find out where he stands and Leia is a porcupine, every time she gets close I'm waiting for her to strike. Their dynamic is potentially more important to the plot than the trigger of Helen's accident.
The story implies there is unresolved business between them, 100%, but the tension can be Leia wanting to punish him, using his male 'weakness' against him (as Vanessa would say), but she could be completely uninterested in a sexual aspect of punishment, and solely interested in revenge. She says "how dare Monica take what's mine" but then tries to push William into seducing Monica. This is far more a cold strategy. The emotional outbusts at the police station and at the club feel as likely a response from an abandoned twin sister (who had a mental issue that destroyed her physical health over he twin's absence) as an abandoned LI.
There's nothing that really confirms Leia sees William as an LI right now, and from William's side, is he trying to fix his percieved guilt, or rekindle the flame? Either reading makes sense, each has its own peculiarities. I'd say that when William is talking with Dylan or on his own, and in the scene with Katie in the kitchen after the pooljob, we see calculating William, who is not an idiot. That William is there the whole time, the asshole buffoon is partially a mask (although, yes he is still an asshole at plenty of times).
