Wait I wanna know what Shannon's original implied origin story was please tell me
First extremely giant caveat is that this is simply my own pet theory, based on what I think makes the most sense from what they gave us about her in s2 and s3 and not like... official or anything. I'm toeing a line between pure head canon and reasonable interpretation based on close reading of the text.
Second caveat is this requires acknowledgment that the powers that be clearly do not have any sort of show bible, did not fill out character sheets for their OCs like any good middle schooler would do, and actually don't know a lot about their characters to start with and just make stuff up as they go along and also change their minds and retcon stuff on a regular basis (like the headstone birth date making them both extremely young).
Which actually, that's my third caveat: accepting that they weren't always supposed to be super young to start with. To me it's pretty clear they were assuming Eddie was Ryan's age in the early seasons. Or at least not as super young as has been established recently.
So my actual REAL answer is that I don't think they gave her backstory any thought at all and had no clear picture for any of it. Just that they were estranged because she had to take care of her dying mom and he was in the army.
But, I think perhaps it's implied that Shannon wasn't from Texas. I think she was from LA and they met in LA and she moved to Texas to be with him. I say that based on a few things.
One, her saying that Eddie left her "alone in Texas with a baby." To me, this is a weird way to say it if it was her home state. Nobody would refer to their home state by name like that. They'd say "you left me at home alone with a baby" or something similar, but the "in Texas" feels pointed to me.
Two, the fact that her mom clearly lives in LA, which would be really fucking weird if Shannon was actually originally intended to be a teenage mother, and her dad clearly is not around. But if Shannon used to live in LA with her mom and moved to Texas to be with Eddie, that makes more sense.
Three, the fact that Shannon is so completely alone in Texas. Granted we don't get much insight to her life, but how much she is forced to rely on Eddie's parents and the everything else about the whole situation implies that she has no social safety net in Texas besides her in-laws. She has no friends or family of her own, seemingly. If she was from there, I'd think she'd have at least one other person to help her out at least a little.
Four, the fact that she stayed in LA, once she got there, and that she was so intent on getting Eddie to also move there with her. That, to me, implies a familiar comfort with the location that could be construed as it being her hometown.
Five, the fact that Eddie has family in LA. Both Abeula and Pepa live in LA. I'd say it makes a lot of sense for the back story there to be that Eddie's Abuela moved to LA from Mexico with her husband and kids and settled down. And then Ramon moved to Texas for petroleum engineering, either school first and then work or just work after schooling, doesn't matter. But that would give them a way to meet. Maybe he used to stay with Abuela for extended amounts of time during his summer breaks in his youth and she lived nearby or something similar. We know that Abuela had the chance to tell Shannon embarrassing stories about little Eddie, so they did spend time together.
And that's pretty much the gist of it!
Since it took me 84 years to answer this, I'll throw in another pet theory for free:
I'm half convinced that they added the divorce date in 2.17 at the last minute. Shannon is wearing the same outfit she's killed in for the pancake facetime call, and the date happens in between those two scenes.
It's entirely possible that they figured nobody would notice that fact and didn't want to do a whole new wardrobe change for one facetime scene. But I do often wonder if originally the date wasn't there and they added it to make it so she was for sure not pregnant when she died and also add in the divorce angst for Eddie.
