A Micaiah with close counter can brash assault melee units?

Uum... Brash Assault only works on initiation against units who can counterattack you, so obviously not; unless the melee unit in question has DC, BA wouldn't work, in which case your CC is still irrelevant.

No; you need to initiate combat for Brash Assault to trigger, and despite Close Counter you cannot initiate attack on enemies in adjacent tiles (1 space away from you).