[MonBon]

This sounds so cool! But unfortunately I should be doing homework instead of properly investigating so here's my guess:

The Murderer: Lady Isabelle Molatto
How they did it: She distracted the butler by hiding the wine bottle he needed, while the butler was in the cellar, she poisoned the steak
Why: She is pissed off at Bosney and she's working with Tavish. Having Tavish be exposed by Bosney would ruin her as well. Tavish's wife wants Tavish's business for herself and would do anything to get him out of the way, and that includes selling him out to Treylbach. Since Tavish's wife and Lady Scrimshaw are at odds, what with Lady Scrimshaw holding more weight in her words than Tavish's wife, the only one who can actually ruin Tavish and thus Lady Molatto is the one investigating him: Treylbach. She hopes that with the threat of Treylbach being removed, Bosney will be less likely to pursue the Tavish lead due to a murky history with Lady Molatto and the poor opinion of the weight of anything Tavish's wife would have to say. Manstrode of course would follow whatever Lady Scrimshaw had to say.