This runs into 2 issues, the musical one is the rather right-sided character, as in it has a Rhinelandish feel (but really, look at his teachers as listed, couldn't be more French). Still, there was a powerful internal debate about Wagnerism and Schmitt's orchestral works do land on the darker side. Many of the composers listed on this thread were dedicated Bretons, and Schmitt presents a flavor of Germanism, I think.
mparta, Issue 1 is the Germanic quality of Florent Schmitt's music, how do we interpret that? I'm glad you separated that from issue 2, Schmitt's pro-German collaboration in World War 2. There are French late nineteenth century orchestral composers whose music has German, specifically Wagnerian qualities, but who were not fascist, anti-semitic, or German-supporting -- Chausson and Magnard come to mind.
Schmitt was born to ethnically German parents in the region of Lorraine, which as a result of the Franco-Prussian war became part of Germany along with Alsace in 1870, the year of Schmitt's birth. (By contrast, Charles Koechlin was also born in Lorraine but to French parents, who took off to the west and France immediately after the German acquisition.) Schmitt's advanced education in composition was with Massenet and Fauré at the Paris Consérvatoire -- both very French composers as you say. But Fauré had learned his Wagner well from Saint-Saens at the progressive Niedermeyer School in Paris. And he did not impose his own style on his students, so that an outstanding talent like Schmitt (who may have been influenced by German orchestral music in score-reading, performance, etc.) could have proceeded on his own path. As for landing on the darker side, there were in the late nineteenth century any number of composers from both France and Germany, not to mention poets, novelists, dramatists, painters, and thinkers who did too. You mention the pro- and anti-Wagner polemics in France, another factor. I'd never thought of Brittanny in northwest France being at an opposite pole from the Rhineland (Alsace-Lorraine, etc.) culturally, but of the French vs. German issue there is no doubt.
So all I can suggest re Schmitt's dark side are some possibilities: his temperament, the spirit of the time, sensationalism and competition, Nietszche, other ideas?
