nLockTime transactions, how do they persist? Are they broadcast before they are valid?
The release notes for Bitcoin Core (Satoshi client) version 0.9.0 states


  Accept nLockTime transactions that finalize in the next block


So it will just discard other nLockTime transactions. Since most miners run bitcoind or some fork of bitcoind, it is unlikely that an nLockTime transaction will persist for a long time.