[Suicide Junkie]

1) You need to build a sensor. They can be components in sats/ships/bases or even on a facility. But you do need to construct a sensor.

2) Cloaking and detection is system-wide.

3&4) Combat will only start if you are in the same sector AND can see at least one enemy ship. (Or they can see one of yours)
Once combat starts, all vehicles present will decloak and join the battle.