I'm having trouble following Acxa's love life, so first she had an unrequited romance with Zethrid and now she's with Keith?
Okay let me be literal here - Acxa in show has no love life. Or at least, we have no explicit confirmation of a love life. All of these are headcanons based on what we know of the characters and what we can glean from the story told. And then extrapolating from there.
The idea behind zethacxa comes from certain interactions in the Grudge, specifically where Acxa tries to convince Zethrid to stop the hostage exchange with Keith. There, she promises that Zethrid could can and should change and if she does she can be "with me[Acxa], and Ezor."* In of itself Acxa says she cares for Zethrid, and Ezor too. But the interesting thing is that Acxa puts herself first. Before Ezor, who you know, is Zethrid's wife.** On one hand, this is meaningless, on the other hand, Acxa is letting out a Freudian slip - implicating that she is (or more accurately should) be a higher priority for Zethrid to change over her own wife.
It could also be cause Ezor left Zethrid on bad terms, and thus Zethrid and Ezor aren't talking (I mean, Zethrid is taking it out on Keith + Acxa instead of trying to talk to Ezor) so maybe Acxa operates on that logic. But if that is so, this means that somehow Acxa, who is responsible for blowing up half of Zethrid's face, is somehow in a better position to connect with her emotionally says a lot about everyone involved.They are so close you all. Either way, Acxa is measuring herself in an equal if not higher level than Ezor in importance for Zethrid, which imho, speaks volumes. And Zethrid also tells Acxa she's "too far gone" which is meaningless really, but it's the type of thing you'd tell someone whom you care for, if at least at one point.
Recall also that Ezor was supposed to have died as well, as one writer on the show admitted pre-s8 that Ezor was dead, but in response to backlash the undid Ezor's death. So in original canon, Zethrid was going to run away with Acxa, and lo and behold, at the end of the episode we have this:
Acxa has and will not give Zethrid any peace cause she cares enough for Zethrid to be with her until she gets better. Your ship could never.
But what does that have to do with past Zethacxa? For that, we look to past canon. Acxa and Zethrid always had a contentious relationship throughout the show. Both are quick fuses, but triggered by different things. Zethrid sought violent thrills, Acxa wants to stick to the plan. Acxa is pissy over incorrect information, Zethrid does not care. Zethrid plans for long-term self-sustainability, Acxa will stick to their current course in hopes of achieving political change. Anything Zethrid does sets Acxa off.
Conflict is a part of their dynamic and since s3 it's been foreshadowed that their arcs will go to a head this way. Now with that one line from the Grudge in mind, reading this animosity shifts slightly. It would be fun, if sometime before the events of the show, if they dated, broke off cause they were to quote Acxa "so full of hate and rage" and cannot carry a normal relationship (I mean, look how zethzor turn out by the Grudge, and Zethrid is older then), and then cause life hates them, they both get recruited by Lotor. Ironic. Their squabbling becomes exes drama. Zethrid is unphased by the shit Acxa pulls in s7 for ex, whereas Ezor took Acxa's leave as personal insult. If Zethrid is used to Acxa being Like That, it would make sense.
Alternatively, Acxa is very emotionally repressed, which given her whole thing with Keith, is a completely sound read of her.
So yeah that's the tea on Zethacxa.
With Keith, I don't really have to explain. Acxa hyper-fixated on him cause he saved her life (She was "born and bred in war" and every person was for themself, so Keith taking his time knowing she's the enemy to save her touched her personally.) and cause he has a freedom to achieve his plans/ideals in way Acxa could never imagine herself doing at that point of time (act 2). She hated him. She cannot let him die. From this, a more mature and self-actualized Acxa could develop an interest in him in act 3 and/or after. It doesn't have to be, nor did the writers want it by the time act 3 rolled around, but there's food there.
In other words - Zethacxa, break up, then kacxa. Or zethzoracxa polycule + Keith fourth wheel. Or very homoerotic intense Zethacxa friendship then kacxa. Or consider - Zethezoracxa but Acxa also has a thing with keith on the side. idk take your pick
*bear with me in that I don't have quick access to s7-8 cause it was never released in hard copy and I do not have Netflix. So a paraphrase from memory
** Yes they're married. To quote Avatar Korra - "You gotta deal with it!"