After seeing which of my characters gets stealth, can my opponent change which of their characters is attacking?
[Source: WotC]
No. Hiding Spot isn’t revealed until after your opponent locks in their choice for attacker.
What happens if someone has Brave and Stealth at the same time, like if I played Hiding Spot on Blitzwing while he was in tank mode, do they cancel out, does the newer stays take effect?
[Source: WotC]
They cancel each other out. An attacker is not able to fulfill both conditions, therefore the attacking player can choose a defender as normal.