Are the options --single-instance and --console not working together ?
No, they don't.
When you run the GUI version of FreeCAD with --single-instance then a server on a local socket will be started. If you start a second FreeCAD instance with --single-instance it tries to connect to the local socket server and if that succeeded it will forward its program parameters to the first instance and terminates itself.
In order to make this working a running event loop is required. However, the console version of FreeCAD doesn't have an event loop running and thus the single-instance mode isn't supported there.
the script is not launched on the existing instance. Is this also not supported?
If the second FreeCAD instance cannot connect to a running server it won't terminate itself and thus it won't forward its program parameters to this other instance. Again, because in console mode an event loop is never started it's not possible to connect to the first FreeCAD instance.