You can't countercharge a hidden model that is charging you. We all agree on that. Brace means essentially always being able to strike first in CC, with unhidden opponents. I think the rules are only bent by the special abilities.
In this case you have two models bending the rules from special abilities. The Unseen Assailant always strikes from hidden, but a perma-braced model is technically always holding a wait action, which you use after the surprise of the hidden model striking.
I've seen this come up before when my Wolf Clan Scavenger snuck across the board and struck a waiting Troll Deathseeker on wait, from behind!! By the rules, a model backstabbed like this has to use it's wait option to turn and attack the model sneaking around.
That's when my Wolf Totem jumped the Troll when it was turned around with no wait counter!!!
It plays very well this way, and I think it's by the rules!!