] I find this whole episode creaks. It seems as though it's a part from a melodrama that's wandered into S&S by mistake. Am I alone?

Sorry to quote her again, but Margaret Drabble certainly does. She says:

"The story of Colonel Brandon's early love, Eliza ... belongs more to the world of "Clarissa" than to the world of Emma Woodhouse, and in her later novels, Austen was much more wary about treating such extreme subject matter - perhaps wisely, as the Eliza story is much less plausibly handled than, for instance, the low machinations of Lucy Steele."