why describe a man as a governess?

This is a tough one. One hint is the reference to the Borstal prison system. Borstal was a prison in Kent, where young offenders were first segregated from the general population, and became the name for juvenile detention centers which sought to separate young offenders from older criminals to avoid the corruption of first time offenders by older, hardened criminals. Borstal became a common noun meaning any such specialized, juvenile prison. Since the reference is to juveniles, it could be that the author uses the term governess in the sense of a woman who is responsible for the care and education of children, and applies it to Thorle to suggest that he was a sort of high-class nurse maid to juvenile offenders. (Note the use of the term nursery--governesses educated children in the room of the house called the nursery). Chelsea was, at the time that Munro wrote (and still is for all that i know) an expensive and high class neighborhood in London, near the fashionable (or then fashionable) districts of Belgravia and Kensington. The overall picture i get is of a dilettante, someone who plays at his professed interests, but who does not actually labor effectively to accomplish his stated purpose. The phrase ". . . evanescently popular amid a wide but shifting circle of acquaintances" suggests that people were impressed with his devotion to the various causes named, until they got to know Thorle better, and were better able to see him for all talk and no action. Chelsea-bred religions, in that interpretation, would be the latest causes fashionable among the wealthy and idle people of that wealthy neighborhood in London, where religion is not actually used as a descriptive term, but rather a cynical description of people whose motives profess to be what they are not. In other words, Thorle, and people like him, espouse causes because the are popular causes but causes for which they are not willing to actually work and dirty their hands.

It's a simile.

Trowel-workers delve superficially, governesses manage nurseries and window-dressers attend to shop fronts.

Ask yourself why you aren't enquiring why he's being likened to a trowel-worker and a window-dresser.

The author is not using similes, the author is using metaphors. The entire passage is metaphorical.

Yes metaphors , similar to similes, figuratively speaking
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If you can't distinguish between metaphors and similes, why are you answering q question of this subtlety?

Because instead of your supposition viz.



I believe the description is merely a figure of speech and you have tried to crack the nut with a sledge hammer.

Quite simply...Thorle was probably effeminate.
Here is a quotation prior to the one given.
From The Unbearable Bassington Cp14 by H.H. Munro

I believe you are desperate to defend your indefensible use of the term simile. I know this member to have a very good command of English, so i was trying to give him or her the sense of the metaphor. You seem to have attempted to repair a watch with a hammer.

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Lizfeehily, a simile describes things which are alike--he is as fierce as a lion when defending his beliefs. A metaphor describes someone or something as something which it patently is not, to create a vivid image--he is a lion defending his beliefs. The first constructions says he is like a lion; the second construction says that he is a lion; he is not a lion, but saying so to give force to the description. Munro says that Thorle is a governess, not because Thorle is a governess, but to give a vivid image of Thorle's behavior--it is a metaphor.

Thank you Setanta.