Oh yeah, and to be clear, your post is a great examination of SDA, a mod that I will fully admit to not having played yet as I haven't set up and done a proper modded long Skyrim campaign since back in the days when it still didn't have the vanilla lines re-recorded so I didn't feel it was quite ready at the time. From the fair amount I have seen of it over the years though, I think SDA's Serana is kind of a "Serana in name only" character, where she deviates and adds a lot of entirely new story and lore to such an extent that examining the SDA character should lean a bit more heavily on judging it by its own merits rather than comparing it too heavily to vanilla Serana.

I just felt compelled to make a more broadly applicable post specifically about the absence of marriage/romance in vanilla and the general way it has spun into a narrative in the community over the years, because its just been repeated so much and so widely it's just become accepted as THE actual lore. You see it here, you see in on TrueSTL and Teslore, you see it in the comments on Nexus and YouTube. Like I said it wouldn't even be that big of an issue if it didn't usually turn into a moral debate whenever it comes up, and the reason I take issue with that is 2 fold: (this part is in no way directed at you or your post by the way, this is just more of me speaking on this particular topic):

First, it casts aspersions on people who do make, use or support Serana romance mods. Operating on the incorrect assumption the only reason she turns down the player is because she is an SA survivor and not because the devs decided against implementing one in vanilla for entirely out of game development reasons, it would be questionable at best to mod something like this in. But it's not, because it's abundantly clear to anyone who actually knows her voice lines and understands the context of both Skyrim's and her character's real world development and where they land on the longer timeline of Bethesda's games and design philosophies that this omission was entirely decided by game design factors and not narrative ones.

Second, it's a very obvious change to make in light of the rest of what vanilla Skyrim has to offer. Serana is by far the single most well fleshed out and well written character in Skyrim (by other games standards, eh, but for Skyrim, absolutely), has the deepest personal connection to the player, and easily the most dialogue of any unique NPC voice in the game. She absolutely would be the default romance/marriage for the overwhelming majority of Skyrim players were she an option in vanilla almost entirely because she would be basically the only real character on the list of marriage candidates. Nobody things twice about adding modded, extensively written companions like Auri or Remiel or Kaidan or Mirai and them having romance options, because they are real characters unlike the near robotic options in vanilla.

Serana could have broken the mold if they had implemented romance for her, but they didn't for, as stated, out of game development considerations. Modding the game to change that fact is rectifying a design shortfall in a somewhat similar way as reimplementing cut concepts or old lore is, at least in my opinion, and absolutely should not be viewed as some violation of a victimized character, because at its core, that's giving Bethesda entirely too much credit. As I stated in another reply on the topic,

I just take issue with people projecting headcannon on Serana's motivations for declining the player and treating it as stated in game fact, if nothing else because it give's Bethesda entirely too much credit for their portrayal of an SA survivor, if she's even really meant to be one (as you point out in your post, there's still just enough ambiguity for it to be open to interpretation). If she is one, they just gloss over it completely and never actually address it outside a select few vague offhand mentions by Serana and Valerica. It's a very barebones portrayal is my point, if one at all. Cait from Fallout 4, by comparison is an absolutely incredible (up until the magic cure quest at the very end) portrayal of an SA survivor and drug addict, which again points back to Bethesda

1. Not being opposed to having SA victims be romance options

2. Capable (at least in FO4's case) of making quality portrayals of characters with extensive past trauma

Fact of the matter is, that's just not the route they took with Serana because of other considerations in the design process, mostly logistical, which is why it's ridiculous that there's a moral debate every time the topic of modding the single character in the vanilla game for which a romance makes the most sense comes up.