To be honest I see it all as sloppy contrivances to answer: "How can we make Fate Mordred an exact lookalike of Artoria while also preserving the (real life) established legend of Mordred being Arthur's son to Morgan La Fay?"
Even though it all makes sense in the end, it's still very obvious that they just came up with something on the fly to make it work with their already preexisting narrative of Artoria. It's just unnecessarily complicated.
@Norainhere
Yes, Mordred is definitely obsessed with her. Her bio in Fate/GO says that you shouldn't praise Artoria in front of her while at the same time don't insult her either. Mordred has the utmost respect for her "dad" but also really hates her for not acknowledging her. Her noble phantasm is Clarent Blood Arthur: Rebellion Against My Beautiful Father for pete's sake.
Fun fact: There's a yuri pairing for Artoria that involves Mordred being the kid in the family. It works because in this pairing the Artoria didn't stop from aging and actually looks her age this time (it's the lancer version of Artoria).
https://pbs.twimg.com/media/CtrcEXrUAAAshi3.jpg:large