Perhaps it's unclear whether it was an accident or intentional. He took her for a walk and she fell (or was pushed and slipped during another argument) either on the rickety bridge or off the adjacent cliff you added. Dialogue with Marc would need to be Orable was responsible (even if only because without him she wouldn't have been there) rather than her killer. Orable claims an accident but the locals have their own views (based on their personal views of Orable).. Perhaps Orable and the tavern keeper could have dialogue, one for each side of the thing, which if you have discovered either gives the dead dialogue for Marc otherwise left to be discovered later?
Isn't that a bit out of character for Rachel? We know that Orable fancies her but she wants nothing to do with him; he's already been violent and you've seen the bruise; and she won't even go to the beach to see Milon unless you threaten Orable. So she's certainly not going for a walk with him to the far side of Martinique! The idea of a two-stage investigation makes sense, though. You could ask Orable how he's getting on with Rachel, he says she died in an accident, then you talk to someone else who tells you it wasn't an accident. After that, you might want to kill Orable. Partly to avenge Rachel, partly to atone for your role in her death, and partly because you don't want him on the beach when Marc arrives in case he tells Marc who put him up to it...
I haven't added a cliff (apart from the bit in the bridge location) but that gives me an idea...
Incidentally I am thinking of giving Marc a role in redeeming Baldewyn's store for a strand where it is not under duress of colony capture - he has knowledge of the goings on but then of course disappeared. So when he is "found" he can give testimony against Arnaud to the Governor (under pressure/suggestion from pchar) which helps replace Baldewyn.
The snag there is timing. Marc disappeared about five years ago, so if he knows about the store takeover then that must also have happened at least five years ago. Loansharks don't give you anything like five years to pay back a debt, so why would Arnaud wait that long before chasing Baldewyn for his loaned money? On the other hand, if you don't hire Marc then he goes to the store, presumably for a job, where he could uncover something incriminating. If you do hire him, you could perhaps ask him to detach from the ship, work for Arnaud for a while, and do a bit of snooping.
I also wonder if Sabine wanting to sail is linked to a crush on Marc really (He's the Captain in her dream) who has vanished although it remains unspoken (is she hoping to run into him) but that is another reason she doesn't want to be married off.
There's no need to tie all the sidequests on Martinique together.
Besides, this won't work if you complete the Blacque family quest before starting Baldewyn/Sabine and choose not to hire Marc; Sabine won't need to go to sea to look for him if he's standing there in front of her! The existing motive works well - Sabine wants to get away from her father and from being forced to marry his choice of husband, and sailing away is a good way to do it.
As for the cliff. The stock game name for both island and town, still used for coding, is "Falaise de Fleur", which literally translates as "Flower Cliff". And the old Carib name for the island, Madinina, translates as "Land of Flowers". At present I've set the bridge to lead to a simple jungle path which leads to another piece of jungle with a T-shaped path - if you're doing the "Crystal Skull" quest, this is where the Carib warrior finds the skull. One branch of the T leads back the way you came. The other two currently lead to each end of another simple jungle path location just to have them go somewhere. I now plan to replace that with a cliff location based on "Muelle_passage", using the flowery grass texture, and named "Falaise de Fleur" - not a place-holder variable like the town and island, but the unmodified name.
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That'll be either Alkella or Nathan Kell. Don't shoot the messenger here! 
