legamus.eu

Yes, what we have is just fine.
Square Eggs and other Sketches was published posthumously, so the biographical part is probably not by Saki either. Just check if his death is mentioned
No, leave it as it is, just the normal prose. Even if it was the original edition, we are fine to pick an choose individual stories, we only don't want the text to be changed.

The biography was by his sister, and googling did not lead me to her date of death. So I agree. I guess that is one problem with not having an equivalent of Gutenberg to rely on!