You may have this information already, however, I hope this is helpful.
In his book "The Story of Edinburgh Crystal" H.W.Woodward tells that Stanley Jenkinson (third generation owner-manager of the Edinburgh and Leith Flint Glass Works) visited Paris in 1915 "to recruit French workers, later importing craftsmen from Italy and Spain, one being Salvador Ysart". Frank Andrews in "Ysart Glass" says that George Rae, the company's agent, made all the arrangements of the family's travel from Paris to Edinburgh.
Another book I trust is Victor Arwas "Glass: Art Nouveau to Art Deco" and he writes that Salvador Ysart was a Catalan glassmaker from Barcelona who emigrated to Marseilles, France in 1909, then moved to Lyons and then to Choisy-le-Roi near Paris before going to Edinburgh in 1915. There is no mention of Nancy.
In his later book "The Art of Glass" Victor Arwas states that Salvador Ysart worked in various glassworks in France including Schneider.
There is a detailed history of the Schneider brothers in Edith Mannoni's book "Schneider". She wrote that although the Schneider brothers appear to have had a retail outlet in Paris around 1911 (the year Ernest Schneider was dismissed from Daum and a year before his brother Charles was dismissed) it was not until 1913 that they bought an abandoned glassworks at Epinay-sur-Seine under the name "Schneider Freres & Wolff" and commenced production on November 25th 1913. The following year, in August, production ceased and the Schneider brothers went to war. Their sister looked after the plant so that it was ready to fire up again in 1917 to make glass needed for the war effort. So if it was the Schneider works where Salvador Ysart was employed, he worked there only a very short time, and he was not working there in 1915 when he was recruited by Jenkinson.
Frank Andrew's book "Ysart Glass" tells us that Salvador was born in Barcelona in 1878 and started at a glassworks at the age of 10. When he moved to France, the link to Lyons is confirmed because his fourth son Antoine was born in Lyons in 1911. When war broke out in 1914 Frank's book records that the Ysart family were in Epiney-sur-Seine and Salvador was a glassblower at the Schneider glassworks.
Apart from the four books I've mentioned, there is a reference in Frank's book to:
Ernest Schneider-Bernard in "Charles Schneider-Charder-Le Verre Francais: Glas Des Art Deco", Christiane Waldrich, Munich, 1987 p 26.