If Mira was a calculated and cold blooded person who could easily manipulate people, fake a personality, etc. she would have picked a role on the ship safer than 'expendable slave'. And, absolute law of mariners, especially among smugglers is that the captain sets the course. So, any theory other than fate or an accident would require the captain to be controlled by Mira.
That isn't outside the realm of possibility and the captain could have been manipulated in several ways. Even assuming that the insurance thing was his own idea, the specific location he wanted to use as a dumping ground to dispose of the bodies afterward could have been something fed to him. I agree that regardless of what the case turns out to be, I doubt the captain was fully involved.
Mira hiding among slaves, however, isn't so crazy. Given this would have been some time after the war on witches and the kingdom's general stance against witches thereafter, it makes sense that Mira would have wanted to lay low in an attempt to evade capture by anyone who might know she was carrying the heart.
I'm not even suggesting Mira was faking the personality she showed to Julian and I think the way he reminded her of her son was genuine. My theory in short is that everything from the plan to go on the ship with 100 slaves to an area near the sea goddess' domain, to the plan of having those slaves be killed was part of a plan. A plan which I believe included altering the course slightly under the cover of night to ensure that they would end up in the right place to turn murder into ritualistic sacrifice.
I also think Julian was not part of the plan until he was discovered as a stowaway and that the ship's crew weren't privy to the full plan and only thought they'd be getting into barbaric insurance fraud.
For me there are just too many coincidences here:
*an experienced crew of sailors was actively trying to avoid going through a specific area but ended up there anyway
*the plan to kill 100 slaves conveniently lines up with making a proper sacrifice to the sea goddess to gain her blessing
*the king has her blessing
*Mira just happened to have the heart and passed it on to Diana
Now I could be proven wrong in due course but I'm having a Doakes moment here.