The Trial by Fire didn't have a restriction on the number of attempts. So long as one was alive, they could challenge the Trial by Fire as often as they wanted.
In other words, it was just a high-level training ground that offered great danger and rewards.
Vaan naturally wouldn't charge to the end of the second stage blindly; he planned to test the waters and adapt. Only then would he advance.
Astoria realized Vaan's intention before she stopped objecting.
Despite knowing Vaan could think and care for himself, she still worried for his safety and objected subconsciously.
It didn't matter how capable he was; she would still worry.
Vaan was an invaluable asset to humanity—or so she constantly told herself. However, she had to admit that it wasn't the sole reason for her concern.
Whether she liked it or not, she had feelings for him.
