Most answers to your questions can be found either in my book J M Barrie & the Lost Boys or else Mackail's 1941 biography, The Story of JMB. George had expressed a wish to be buried where he fell, and although this couldn't be precisely granted, I'm sure his brothers would have objected to his body being repatriated, even supposing Barrie ever proposed such a thing (which I sincerely doubt). I also don't think anyone would have wanted to guild the lily of his simple grave, more or less identical to those of his comrades. Second Lieutenant (pronounced "lef-tenant" by English speakers) is a junior commissioned officer military rank in many other countries besides Britain and the USA ...
... and yes, all the brothers smoked. Daphne du M put their emphysema down to their glass-blowing French ancestors.