Changes with respect to Blind-Fight.
1. Yes, but rolled twice for melee attacks. You could treat this as 25% miss chance.
2. Yes for ranged attacks, No for melee attacks.
2a. Yes for ranged attack, No for melee attacks*.
3. No. There is no penalty for the attack roll.
4. The attacker cannot take attacks of opportunity against concealed opponents (which would be everyone). He also moves at half three quarters speed and thus cannot take a 5ft-step (big penalty AFAIC). There are various skill/ability check penalties.
Also, you don't know where the enemy actually is, you have to pick squares to attack into. You can also make a listen check DC = move silent check of enemy to know generally where they're at, DC = move silent check of enemy +20 to pinpoint their square