there is very blatany subtext within canon pointing to their relationship being a thing, it is not once directly stated that they are together legally, it is not an established fact within canon, so if they ever decide to change that in the future, it wouldn't be considered a retcon,
This is very common, but you don't seem to be using the word subtext quite right. Subtext is a plot device used to convey an idea implicitly, so it's a part of the narrative that is not explicit, but it's still part of it and therefore canon. Something that is only hinted at is NOT subtext going by the formal definition, but this is not the case for Nanoha and Fate's marriage, their family and adopted daughter, or their active sex life together.
because subtext does not mean certainty.
I don't think that's relevant, neither text nor subtext can provide certainty, both in literature and reality we only have access to what we call reasonable certainty. Certainty or absolute certainty only exists in logic, math and systems derived from those.
I won't pretend i understand law, but i don't see how Fate being Vivio's guardian is a big point, as she is also Caro and Erio's guardian, and while she may refer to them as such, they are not formally part of the Harlaown, Testarossa or Takamachi families.
Fate becoming Vivio's guardian was one of the most important things that has happened in the franchise, as it confirmed their already hinted at marriage. And Fate also had the intention to be Caro and Erio's guardian, but she couldn't do so because she was too young at the time, so Lindy adopted Erio, and the situation about Caro I don't know if it was ever clarified since she still has her biological family alive, but everyone seems to believe that Lindy adopted her along with Erio. That's what happened legally, but Fate's relationship with them in canon is somewhere in the middle between big sister and mother figure, and Vivio in one of the later sound stage says that she sees them as siblings too, which was cute, even if they rarely can meet.