All ships are required to show only the lights as per COLREGs and none other.
The masthead light should be visible at 6nm and the side lights at 3 nm. That is enough for visual detection and collision avoidance.
Stealth ships are only designed to reduce radar signature, not completely erase it. A small radar signature used in conjunction with electronic countermeasures helps a warship in deflecting incoming missiles.
I cannot really explain why the USS Fitzgerald and USS McCain could not avoid the collisions. You have to wait for the investigation reports.
However, I can say that a clear understanding and a strict adherence to COLREGs is necessary for OOWs. It is evident that a lot of navigators develop incorrect interpretations of COLREGs that solidify over time.