I'm wondering if we're being taken on a very twisty path with this story. Otis may well be Wickham-like, but only because he wants Miss Lambe's money for political ends. She mentioned that he belonged to a group of some kind - I assume it's an anti-slavery pressure group of the time or purports to be such. I imagine that, even if she is below the age of majority, her money might become available to a husband should she marry. This may be his real motive and Sidney knows it, or even that the political group is not genuine?
As for young Stringer - does he seem too good to be true? Playing devil's advocate here, but what if he's not as nice and decent as he seems? At the moment we suspect he's the good guy who won't get the girl - perhaps he's not the good guy at all?
Maybe grumpy Sidney is the only good guy in this - just a thought!