So watching old episodes clips on YouTube I made some sort of parallel (could be the late hour though).
Buck was in-love with Abby, but she didn't love him back, she stuck it out with him until her mother died.
He was her emotional support boyfriend, until it wasn't enough and she had to leave to find who she really is without her mother's illness looming over her.
So she let go of Buck, but didn’t completely break up with him - my assumption was that she didn't know how things would go, so he was a sort of safety net, not because she loved him, but because she knew he was there to catch her.
In between, Eddie comes along, neither dates anyone until late season 4, and even that is because Eddie is encouraged by Bobby to do that. Notice no one else encourages him to do so, because it's clear that he is not available, he's just not seeing it yet. ("A*a who?" " It's like it a*a's and Eddie's kid" etc.)
Also, I'm pretty sure that until that talk in 5x02, Buck and Eddie never talk about her (while they're dating - " I thought you two were doing great" meaning Eddie didn't share his struggles until Buck pressed about the panic attack.) or refer to her as Eddie's gf.
The only one referred to as gf was Taylor, and that was said in contempt when she reported about the treasure hunt. And they weren't even a couple yet. (Buck was friend-zoned in that episode, only God and Tim minear know how he made it back from there.)
Now one would think that the boys filled a space for each other until they found a person to replace the love they lost, but that's not the case.
Eddie went so far as to make Buck Christopher legal guardian, should anything happen to him, and only share that detail when he realizes that, unless he makes clear how far he would go to keep Buck integrated into their lives, he may lose him.
In Eddie's case, A*a was the "emotional support", but not in the same way that Buck was to Abby. Eddie is, was?, under the impression that his son needs a mother, and his parents wanted the next woman Eddie picks to check all the traditional boxes that they think she should be.
A*a fit perfectly, on paper, the one thing Eddie forgot was, that you don't marry a woman or keep her around just because your son needs a mother. At the end of the day A*a was ,mostly, a glorified babysitter. She sat with Eddie in the hospital while he was unconscious, she looked after Christopher during the crisis, but nothing more.
She is not integrated into Eddie's and Christopher's lives completely. Ravi have met her for the first time during the crisis and as it turns out it's also her first time in the firehouse, so it stands to reason that out of all of them she may have met only Buck, and that's also, probably, in passing - considering there were no pleasantries exchanged when she and Chris visit during the blackout.
Kind of like how Eddie met Abby in passing, on a call, by accident, hated her, but said nothing other than stepping back with a disgusted/shocked look on his face before leaving to head for the train car.
A*a was a "godsend" during the blackout "taking care of Christopher". But nothing more.
Even the suits shopping, Eddie doesn't want a suit and Christopher is anything but excited about it, but Eddie capitulate for her "You just like to dress me up" and convinces Christopher to play along "come on..."
Neither of them is actually happy, she's just there now, but she holds no significance in their lives. "I'm not his mom, I'm just a friend."
Eventually the one to sort out Eddie's mess is Buck, but Eddie doesn't realize something is significantly wrong with his choice until his own crisis (Abby's was her mother's death) - he gets shot.
He looks at Buck until he collapses, he stares at Buck under the firetruck until he closes his eyes, he asks for Buck and his wellbeing when he sees he's covered in blood, and as he loses consciousness his eyes are fixed on Buck’s face.
At this point I want to point out, that when Eddie has his panic attack, and is sure it's something else, he looks anywhere but A*a.
When he wakes up, he asks for Buck - the smile on his face when he saw Buck run into his room. - The fact that A*a left them alone to call Christopher, even though she could have made thst call for him at any time.
Eddie had his realization, that something is not right, it takes a panic attack, and a near panic attack in the station when A*a and Christopher are visiting for him to sort out what's wrong. And when something is wrong he talks to Buck who uncovers the big picture for him.
Buck knows he wasn't what Abby wanted, he knew it then and it became all the more painfully clear when she returned with a fiancé. - He loved her, but he knew she didn't love him back.
And A*a is in the same place. - she says so herself later in 5x03. (Why she dragged it out herself, is beyond me.)
She was only Eddie's emotional support, but unlike Abby, Eddie already has what he's looking for, and on some level he knows that too. He knew that bleeding out on the concrete floor in the middle of the street, he also knew that when the last sight he chose to see on the firetruck floor, before losing consciousness, was Buck's frightened face.
That's the end of my parallel, I'm super tired and it's like 5:30am after an all-nighter, so I hope it's mostly coherent.
Before I'm wrapping things up and reunite with my pillow I will leave you with one little thought I have - I think that Buck indeed spoke about Abby, but I think on some level he also talked about Eddie too. Because Buck didn't start dating until Eddie chose to do so.
And Buck didn't need his own emotional support (stunted as she may be) if his own rock was still single.
I think Buck knows he loves Eddie (in which capacity is unknown) and on some level it hurt that he was considered "not enough".
That without meaning to, you can see that Eddie's words had impact - "my kid loves her." "Ready-made family". - You can see Buck actually holding back a flinch at those.
So yeah, parallels all around... so tbc?! ;)