I agree with everything you say except this:

The ugly scene in which he says all manner of gratuitously nasty things about Col. B, and eggs on Marianne to do the same, is especially icky.....

I take exception to the bit about egging on Marianne to also slander the Colonel. Whilst I accept that people can be goaded into doing this by the encouragement of others, let Marianne take responsibility for her own words. We already know that she thinks him a dull old man past all chance of romance; I don't think it's Willoughby who is responsible for her holding him in derision. Let's not use that against him, at least.