Hello!
The Ergrek Terminals are instantiated per-player, so each player could get items from them after hacking, even if it was hacked by somebody before.
No, the items do not respawn. It could be done by a mod (with locking terminal back so it will be required to hack it again). There are two scripts related to hacking: "data\scripts\internal\Hacking.js" and "data\scripts\special_objects\ergrek_terminal.js". The latter script is very straightforward and allows making everything you want.
Enemies respawn rate could be also altered by a mod - you could modify the script "data\scripts\global\CoreFunctionality.js" methods "RestorePiratesPopulation()", "RestoreAliensPopulation()", etc.
You could read more about modding and scripting at
Official Wiki.
Regards!
