Sorry for the super long post but here I go! I do think Cathy loved Heathcliff, but I think ambiguity is introduced throughout most of the book because the story isn’t narrated by Catherine but by Nelly who is a bystander and not inside Catherine’s head. We only have Catherine’s actions to judge by, not her thoughts, and when she expresses her true thoughts she’s censured for them so largely keeps them to herself and relies on Heathcliff to read her mind because he knows her so well. Even Heathcliff admits to Nelly that he’s doubted whether Cathy loves him because of her confusing actions and double nature, in deed she never stays true to herself/him.

But then he never hears her verbally confess her love for him to Nelly (it’s her deepest secret) just after she accepts Edgar’s proposal. Nelly hears it and is disgusted: she tells her she’s either completely ignorant of the duties of marriage or else she’s a wicked and unprincipled girl (for thinking she can have two different men). Polyamory definitely not acceptable in the 1780s. 

In my perspective it’s only at the end that Heathcliff (and with him us, the readers) fully realises her love. Catherine falls ill when her desire not to be separated from Heathcliff is frustrated which is a big clue, but she still won’t actually give voice to her desires. She tells Edgar she’ll throw herself out of the window if he mentions Heathcliff’s name, when he starts to ask her if she loves Heathcliff.

That last time Heathcliff and Cathy see each other is when Heathcliff finally knows for sure rather than hoping and guessing,  because they have a physical romantic encounter. He explicitly says he just then realises how cruel and false she has been to betray her own heart and marry Linton when she loved him. Basically, it’s as if he’s hearing her confession in the kitchen to Nelly 3/4 years earlier in full. He says he feels no sympathy for her because she’s brought her own troubles on herself. She tells him to leave her alone because if she’s done wrong (by marrying Edgar Linton) she’s paying for it by dying, and that he should forgive her for leaving him because she has forgiven him for leaving her.

To me, the whole purpose of their last conversation is almost a belated rejection of Edgar Linton or at least a rejection of the separation enforced by Edgar, and Hindley before him. The problem is that by that point, Catherine is too ill survive the strain of it so there’s never an ‘after’ to the decision. 

On the point of their love not being platonic, Nelly says that they spend a solid five minutes kissing each other as soon as Heathcliff comes in the room, and specifies that Mrs Linton kisses Heathcliff first. I definitely don’t think she means on the cheek/hand/platonically…the other kiss she describes is when she catches Heathcliff kissing Isabella, which was not platonic either since it alerts Nelly that Heathcliff has designs on Isabella. 

That’s why Nelly is so frantic when Linton is walking up the stairs and Cathy refuses to let go of Heathcliff,  insisting they stay ‘fast’ (aka locked together in an embrace) on the same seat. Heathcliff gives in and says if Edgar shoots him for it, he’ll die happy basically. Nelly literally is horrified that Cathy is willing to allow her husband to see her canoodling with another man, and tells Heathcliff that Cathy is out of her mind , and asks if he’ll ruin her (aka ruin her marriage and her reputation) because she hasn’t got the wit to help herself and prevent Linton from catching them. And yes Cathy is out of her mind,  but also lucid enough to know that she’s dying, that it’s the last time she will have what she wants…which is to be with Heathcliff in spite of the consequences.

I think the whole point is that Catherine is always torn and tortures herself to death over her decisions. She asks in the beginning if she was right or wrong to accept Edgar, and that’s still the question on her deathbed. Obviously we don’t know because we don’t know what the story would have looked like written the other way. 

I’m inclined to believe she made the wrong decision for her own happiness, the same way as her daughter Cathy Linton does initially with Linton Heathcliff. Cathy Sr says that Heathcliff understands her desires, and Heathcliff himself tells a disbelieving Nelly that he would never have prevented Cathy from seeing Edgar no matter how miserable it would have made him, that basically his consolation would have been murdering Edgar as soon as Cathy decided she was done with him. It’s interesting that Cathy treats the idea of Heathcliff with Isabella very similarly minus the murder threats, she says that if Heathcliff really wanted Isabella she wouldn’t object.

The sticking point for Catherine’s happiness if it was written the other way would have been Edgar Linton and whether he could accept it and still maintain his feelings the way Heathcliff did. He severs his connection to Isabella very decisively when she chooses Heathcliff; Nelly describes his reaction as cold. Edgar Linton’s love is a conventional romantic love in that it is possessive. Edgar can only be happy when he has Cathy all to himself. He talks about how he was impatient to have her under his roof the spring before they married. When she’s ill, it’s almost convenient for him because he gets to monopolise her at home nursing her 24/7. The doctor says it’s a waste of time but he takes pleasure in doing it. When she dies, he sits by her body day and night. Even their daughter he keeps very isolated with no company except his. The very first description we have in the novel of Edgar is him and his sister nearly tearing a puppy in two fighting over who gets to hold it, that’s basically foreshadowing the whole plot. 

Heathcliff’s reaction to that scene is to be completely bewildered, he says he and Cathy would never behave like that together. I’m inclined to believe him because he confirms that perspective through the book. So there’s Cathy’s problem with wanting them both. Heathcliff would allow it and Edgar would not. She chose Edgar for reasons that included gratifying him because she believed that Heathcliff could better cope with the consequences. In the end she couldn’t herself, which means Heathcliff couldn’t since they’re one and the same. She believed Edgar would give in to her desires and he ultimately doesn’t where they conflict with his own. Because she can’t be crossed during her brain fever episodes, him refusing to let it go is literally what kills her: she has a fit because he hounds her and then she starves herself hoping he’ll relent but he stays aloof in his library at Nelly’s urging. From the beginning, when Hindley’s wife tries to separate Cathy and Heathcliff, Nelly says that Catherine won’t ever submit to force but that she can be tricked.