According to BMP 4:
A Safe Muster Point is a short-term safe haven, which will provide ballistic protection should the pirates commence firing
with small arms weaponry or RPGs
and;
A Citadel is a designated pre-planned area purpose built into the
ship where, in the event of imminent boarding by pirates, all crew
will seek protection. A Citadel is designed and constructed to resist
a determined pirate trying to gain entry for a fixed period of time.
The decision to use a citadel vs a safe muster point is mostly up to the owners as it requires a significant modification of the ship, it has nothing to do with unions. Some ships have beefed up muster areas and keep drinking water etc but in most cases they don’t meet the criteria for a citadel My own observation is that citadels are used on the “low and slow” as a lower cost option to hiring armed security
As far as rules of engagement, the Coast Guard has specific language regarding what threshold has to be crossed before the use of deadly force can be used. I don’t recall the exact wording but the ship being fired upon is specifically mentioned as being sufficient justification to return fire and there is no requirement to wait until attackers are inside the rail.