Maybe this is already known or obvious, but I just wanted to add it: I only get this behavior (error messages from NM and a delayed splash screen) when shutting down using Gnome Power Manager - I mapped my powerbutton to "shutdown" - and when I press it it shows the described behavior. When shutting down via System / Quit I don't see the error messages and the splash screen comes immediately. The described changes to the GDM settings did not resolve the problem for me...
Maybe this is already known or obvious, but I just wanted to add it: I only get this behavior (error messages from NM and a delayed splash screen) when shutting down using Gnome Power Manager - I mapped my powerbutton to "shutdown" - and when I press it it shows the described behavior. When shutting down via System / Quit I don't see the error messages and the splash screen comes immediately. The described changes to the GDM settings did not resolve the problem for me...