OK, this seems like a stretch, but just go with us…
It's simple – when conservative Wilcoxes meet liberal Schlegels, conflict is inevitable. The "dark power" here could be seen as a couple of things – at this early stage, it seems to be the Wilcoxes themselves.
We wonder if perhaps Wilcoxes and Schlegels can coexist in the world. They're neighbors now, and it seems like things might actually work out. Margaret and Mrs. Wilcox form an odd bond, and their relationship makes it appear as though the breach between the two families is at least partially closed.
Though it seems like the Wilcoxes and Schlegels seem to have broken off their association after Mrs. Wilcox's death, they find each other again after two years. Mr. Wilcox takes a liking to Margaret, resulting in their marriage. It looks to us (and to Helen) like Margaret has gone over to the dark side – she's fascinated by Mr. Wilcox's air of masculine ability and strength.
After the horrible events of Evie's wedding, Helen banishes herself from England, and it seems like she'll never return. We wonder if the Schlegels will ever be reunited. Have the Wilcoxes and their mode of living triumphed over the romantic, idealistic Schlegels?
After Society has basically crushed everyone – Wilcox and Schlegel alike – a new hope turns up just in time. Back at Howards End, Margaret, Helen, and Mr. Wilcox are all reunited. Helen's baby (notably, also Leonard's child) provides hope for a new kind of Englishness, one that's less torn apart by the problems of older social standards and structures. Even Mr. Wilcox has found a kind of release, and he and Helen are finally reconciled to each other.