Why did she never return?
I strolled a bit on the internet and to be honest I couldn't find the (better: an) answer. In 2010, at the age of 91, the granddaughter (1919?-2010) of Marthe de Florian died. On the inheritance list an apartment was discovered in Paris. An apartment she left in 1940 - she went to live in the South of France - and never went back there. All those 70 years she faithfully paid the rent. For what?
In the apartment an unknown Boldini portrait was found of her grandmother. Worth 2.1 million euro. And a lot of love-letters.
P.S. Somewhere in the weeks/ months to come I'll puzzle on this.