This is definitely worth working. And I definitely have a few comments. Onward! (quotes from you are preceded by a > symbol)
>Whispers reach Oak-Rose as she stands at the window gazing out onto the trees. Furtive, excited whispers; frissons of delighted shock.
First sentence is fine, but the second is not. We don't know Oak-Rose yet, so it's too soon to characterize those whispers. Strike the sentence and let what comes after speak to excitement and delighted shock.
Getting to the whispers, it took a while for me to realize they were coming from women inside the house. So far, the only physical orientation is outward, toward trees. So that's where I pictured the whispers. Maybe Sentence Two could address that. Whispers coming from behind. This isn't a huge deal but it was a bit disorienting.
>Muffled whispers. Spilling from pale, painted faces, concealed behind fans of vellum and ivory, lace and mother-of-pearl.
More about whispers. I don't think they need to be muffled, since they're coming from behind fans. Coming immediately after the black face, I read the painted faces as having been painted to be pale, and so wondered if they might be black or dark but that didn't make sense, so it was another point of puzzlement.
The bit about slavery took me aback. Eventually it makes perfect sense, once I place the scene in early modern or even 19thc England. But coming to it cold was, well, another disorienting bit.
> Lady Oak-Rose was power incarnate. I was transfixed.
Transfixed makes me picture the narrator unable to move. Fascinated, maybe? Some other, more portable, reaction?
>The distraction came
This misses an opportunity, I think. It's here you wish to build tension. Something mysterious happens, and only our narrator notices. But the ominou shaking is made parenthetical by sandwiching in first by Mr Eliot and his background, and then be maybe a bit too much description about the paintings. At this point, is the quality of the paintings really more important than the fact that an apparent (to the narrator) earthquake has not moved them? Focus on the ominous here, and let us feel it by feeling the narrator's reactions.
You could try having "then Mr Eliot appeared" immediately after the quake; a sudden arrival, followed by a bit of background on him. Raise the tension, then leave it hanging, followed by his boorish remarks.
Then, after the fan attack (nice), the moment is undercut by the bit about vapours. Consider putting the focus on poor Mrs Browning, who surely has earned her own fainting spell. The narrator rushed to her. Other women and men can react but in the background. They aren't the point. The point is to narrow us down to the wounded Mr Wilson and the somewhat hysterical Mrs Browning, accusations and protests and damp cloths, then Harker with unexpected and somewhat annoying interruption.
Maybe a little more than one sentence to follow. Maybe something about a slow restoration of order. Maybe Harker awaits instructions, the persistent servant. And only then is the Admiral's body found. No need to be coy about that. Not "The Admiral had been found", especially since as far as we know, no one was looking for him. And definitely not "His body, that is," which isn't dramatic enough for the moment. Could the scream be more articulate?
"The Admiral! He's dead!"
OK, that's a bit much. Scream. Someone comes pelting in from another room. "The Admiral. I think he's dead!" Something to keep us right there in the room, rather than observing from a distance.
And lastly but not leastly. I was sort of ok with the alternating POVs and tenses, which is saying something as I generally don't like that sort of nonsense. But I think I'd stick with it. The present tense sets Oak-Rose apart and hints that she is not an ordinary sort of person. Or not a person at all. It might prove difficult over the course of a novel, but you invented the problem, so you get to solve it. <g>
That's for tenses. As for the POV, I'll say that the switching felt a bit rushed. Not too awfully much, but I'd encourage you to think about each tiny scene as still being bound by the scene guidelines. Let each develop character as well as move the plot. Since the scenes are so brief, the development will be tiny, and very likely not every one will serve the purpose, but don't neglect to treat each as an opportunity to advance character, theme, and setting, as well as plot.
Don't put it in the trunk yet. Oak-Rose deserves more of a chance.