Seems like what you're planning to do is not supported by the engine. You cannot add/remove lighting in a city. Either you have them, or you don't. It's hardcoded.

About one-time build units, it's not a feature of the engine, however you can do it with a little scripting. Use a building as the trigger and this building cannot be destroyed (so it cannot be rebuilt). Once the building is done, it triggers the script which would give you this one unit.