Tatsumi wears a wig
In the anime and the manga, Tatsumi is bald and doesn't hide it. This story's version of him has black hair. It's probably a wig.Sadly Jossed
The false Athena is Seika
Seiya's big sister went to Greece after him, lost her memory and wandered around until the Big Bad found her and decided to pass her as Athena to keep the saints from realizing Saori Kido is the real deal. It'll be revealed in the sequel if one is made.Jossed
The 3 unnamed enemies from the beginning are actually Silver or Bronze Saints
They are often referred as ordinary soldiers from the Sanctuary, but in the manga (and in the original anime series) the Sanctuary never sent ordinary soldiers to eliminate their enemies.That would also explain why they were considered to be relevant enough to have a cameo in the last scene