I don't see any problem with overthinking tbh, and writers are supposed to think about the tiniest details in their story and keep the number of random stuff and useless elements to a minimum. So I think your approach is perfectly OK and you have a good head for reading the literature.
By making overcomplicated, I meant the way you put the different pieces of data together, and I just restated what we know about NnT world till now. However, when Nakaba himself admits that SD as the eldest creation of Chaos is the least known and the most mysterious character to the point that even her existence is almost unknown (tribute to
@FKS for translating the data book portion about SD), it means we can't overlook the details or try to explain all of her actions and decisions using the little data at our hand.
My biggest problem is why she didn't show up sooner to help her partner in crime?
My headcanon for now is that
death of AAs and Escanor, and return of all the graces to heaven was a necessary step for her resurrection, and she couldn't get out of her whatever confined state before that.
I've only made up this theory to give meaning to
Escanor's death though lol.
I lik your idea of a possible confrontation between SD and Arthur. A dialogue between creator and its sweet scheming daughter has infinite potential after the truth about Chaos is revealed.