Saki's style is one of the reasons his stories are such a success - on the whole they are very precise, matter of fact pieces of fiction that adds great humour to the content. Thus given your list of pairs above, Saki's style can be described as: restrained, precise, sparkling, light and formal. It is quickly established that this story is narrated in a very objective tone, that only serves to heighten the humour. Elements of the sparkling wit that Saki employs are evident in the last line, which clearly shows the great comedy and irony in the story as Vera "calmly", without batting an eyelid, tricks her own family with a story she creates out of thin air to explain Framton Nuttel's mysterious rapid escape when he sees the rest of the Sappleton family enter:
Romance at short notice was her speciality.
You might want to think about applying the other adjectives in the pair to the story and considering what the story would be like if they defined Saki's style - it would be a very, very different tale with nowhere near as much impact or shock value.