As the plugin currently works, it tries to find Marvin's database by accessing VendContainer. This is what Apple blocked in iOS 8.3, right?
If @LuCouto is seeing the plugin working with Marvin, on iOS 8.4.3 && iOS 9, that can only mean... that calibre's bundled copy of libimobiledevice, on OSX and Windows, was able to gain access to VendContainer on iOS 8.4.3 && iOS 9.
Why that might be, I don't know. Is it possible that the French version of iOS does not have the VendContainer restriction? Maybe... but then, too, maybe @LuCouto got a special version of iOS from the gods of ereading.