Hmm. No doubt the DD could do formal letters, but presumably she doesn’t really write one to Harriet, given that Harriet is inspired by it to turn up to talk to someone sympathetic.

probably didn't go "Dear Mother-in-law, Peter is great in bed but we've found a dead body in the cellar"

Not in so those very words perhaps, but surely in implication. It all depends on the context of saying how happy Peter is making her! Even goosefeather beds can be mentioned in the context of the mixture of furniture in the place “... dreadful Whatnot and a host of aspidistras, but at least he’d left the good old settles, and a real goosefeather bed right out of a ballad.”

The missing stuff adds to the charm of the book for fic - there’s so many moments where a different POV would be fascinating, or where the reader can tease out of the text all the significant stuff that’s been happening off-page (I am convinced, and one day going to write an account, that we are intended to deduce that it is Bunter who tells the DD at Denver that Harriet doesn’t know about Peter’s shell-shock and really, really needs to Right Now).