Labyrinth ($$$)

2. Then, in one battle against Pyra, she always escapes, even though 1 or 2 of her stacks are affected by Blind. What could be the reason? Inferno has great initiative. But how can she escape while under Blind? It's unclear.

Pyra has the Artillery specialization. If she's advanced or expert, she can escape during the ballista turn, even if her troops are under Blind. Destroy the ballista, and she won't be able to escape.
The same thing happens if she has the Healing skill and a healing tent (I don't remember the correct name).

3. Which mage is best to play with if you have the Titan's Cuirass? K'sIron, Aislinn, Deemer, or someone else?

It all depends on your preferences; play with the one you like the most, as each of them is stronger than the others in different situations.