Bunter was, of course, Sgt Bunter. And how he came to be in Peter's service post-War is described by Peter's mother to Harriet in Busman's Honeymoon. That bit of back story is interesting. Bunter deliberately sought Peter out and stayed with him even though Peter was in such a state of total breakdown he did everything he could to send him away. Bunter is almost Peter's replacement father (dec'd) or elder brother (ineffectual), and also certainly is the person whose reaction to the marriage Harriet worries most about.
I do not think it follows that just because one character is damaged, then all must be in the interests of some abstract fairness. If you look in, eg The Unpleasantness at the Bellona Club, there are examples of those both damaged and undamaged and they do not follow class lines.
But class is an interesting theme. I think Sayers is successful in showing class status in itself not a particularly worthy attribute - especially given the time she was writing. It's not brushed under the carpet (eg Harriet's description of "noblesse oblige" where she compare's Peter to the village blacksmith). But the admirable characters are those who use their advantages to general advantage - detecting, building non-slum housing (presumably following the Shaftesbury example as a forerunner of social housing) with spacious pubs and with fair rents and good maintenance, working outside the home, establishing a business to employ thise who would otherwise struggle to find work, and generally using brains; and choosing friends for their merits (the East End villains are not the best drawn characters but at least they are there), and choosing spouses for love not suitability (both Peter and Mary in contrast to Gerald - whose wife's stuffy class-conscious approach is derided). Peter often drops his title and does not stand on ceremony except when to wield it for his own advantage (Have His Carcass being a good example).
Bright young things and upper class twits are most likely to be portrayed as villains (eg the drug abusers in Murder Must Advertise) and generally as a waste of space. Personal wealth meant not everyone needed to take a paid job, but those who did not have an occupation were shown as not admirable and shallow. There was a similar paradigm with younger working class girls between the diligent and one whose head had been turned by the talkies. Or perhaps Miss W, part of whose woes Miss C attitudes to absence of an occupation.
Harriet's overtly feminist friends like Peter - they see him as an example of a useful person, not typical of his class. They are, of course, part of the same works of fiction, but it is a viewpoint which invites the reader to look at attitude and achievement rather than position.