Where did you get evidence that they died?
Also they cant be Luso's Parents because they have different last names, Marche´s surname is Radiuju or something like that.
Not to mention that marche is blond and Ritz had naturally colorless hair...
Time hasnt passed for real Luso, He comes back at the same moment, same music still playing on the box, same last school day, so Mewt couldnt be cleaning the library, he just got there, saw Luso sleeping and that's it
It's one of the most accepted theories that FFT happens on a distant future Ivalice. Its stories mentions a time in the past when technology was more advanced, airships filled the skies, and references somewhere to some natural disaster involving a floating continent that made other races go extinct and technology to be lost.
Makes sense since you find some quite advanced mooglecraft-like machinery lost in some ruins at a given part of that game.