Q: what's the priority to execute these buy orders, when they will hit blockchain? Same as usual?
A: As soon as the node scheduler storing the scheduled transactions sees the exchange offer as an unconfirmed transaction, it will immediately broadcast your currency buy transactions.
And they will compete with the rest of the transaction for inclusion in the next block, according to the usual transaction priority.

https://nxtwiki.org/wiki/How_Tx_Processing_Works   

Время вроде тоже решает