--override.constantinople=9999999before the fork will postpone the the fork to block 9999999 (verified with a new mainnet node).
--override.constantinople=9999999after the fork will postpone the the fork to block 9999999, roll back to block 7079999 and sync from there (verified with an already forked Rinkeby node, fast sync resumption worked too).
sendare safe against re-entrancies. It is just time until contracts are found which can be attacked. This is not fair and will reduce trust in the Ethereum network. If it is implied (in many documents) that these functions are re-entrancy safe and that suddenly these are unsafe against these attacks this will reduce trust in the network a lot since the way how things work apparently can change over time.