How exactly is it known who gets the next block? are the coins unique? what exactly makes your account different from another one with the same amount in terms of mining?
Complete iteration thru all account lets to calculate the time when a particular account could forge a block. Coins r not unique. The public key does.
Is NXT messaging implemented yet?
Part of Nxt Messaging is already implemented (Arbitrary Messages, Alias System), part is being implemented (Account Control).
Does the open marketplace have a date for when it will be released?
No. Maybe it won't be implemented at all if someone does it by using Arbitrary Messages and Account Control.