Why is it none of the silver saints recognize that Aria isn't Athena? Because all of the old silver saints were killed before the Hades Arc, and Athena has been in hiding on her island due to her arm infection by Mars. Mars is taking advantage of her being a recluse to use an impostor who (Anime Hair aside) doesn't even look remotely like Athena.
Remember how Mars seemed very interested when it was revealed that Kouga had a powerful light element? Now we know that Kouga was actually a darkness user, and that this was apparently a major factor in Mars's plans, justifying his interest. He legitimately wasn't expecting Kouga's element to be light. This might also be the reason for his sudden intervention when Kouga's light element flared up.
A meta example. Remember how the eye catch showed the pegasus cloth crystal and then a faint black one briefly beside it. Kouga's true element is darkness, the eye catch was showing the hidden truth.
Another meta one in the choice of choosing the Roman god of war, Mars, rather than the Greek one, Ares. When it comes to the Japanese pronunciations, there may have been some confusion with the already-existing Gold Saint of Aries.
Putting it back in-universe: he wasn't born a god incarnation, but became one after losing his first wife drove him insane. Furthermore the idea wasn't to be another god fighting Athena for control of Earth, but to establish a new society on another planet—Mars. Him not sharing the Olympian naming theme suddenly makes total sense.
Also in Mythology Ares usually lost and had his rear handed to him on a silver platter, which is to say embarrassingly. But Mars was treated with more respect by the Romans compared to the treatment of Ares by the Greeks, to the point of being THE War God, Athena was demoted as Minerva. So the idea could also have been to establish that Mars was a big threat.
Kouga himself saw Athena taken away by Mars. Yet he's still convinced that going to the Palaestra and winning Saint Fight, conventional means of meeting Athena when she's not in the middle of a Holy War, means finding her.
Now that we know the other Athena is a fake one this one is probably justified
But that doesn't change the fact that Kouga himself leaned that Saori was Athena and saw her taken!' Yet we don't get a single "You guys are Saints. How come you're not freaking out? I thought she was kidnapped!"
Kouga was the only one who saw she was being taken and the other Saints were convinced that the fake athena was the real one and Kouga went because he had a little bit of hope that she was at the Sanctuary.