Since the day after the NGX was released, I have used it in my default flight that loads in the Free Flight screen. To be more precise, I loaded the NGX at the airport and gate of my choice and then saved the flight as the default flight. I never had a single problem with it.

There's no way around this unfortunately. We definitely recommend keeping the default flight as the ultralight

Of course there is, add DisablePreload=1 into the [Main] section of the FSX.cfg then you wont get the double loading of the aircraft if you have the NGX saved as the default flight.

I have currently this option (DisablePreload=1) in my fsx.cfg file, as I use "Xtreme FSX PC" tool that has this option checked in the profile I use in this tool.