Evan,
Usually a white scum indicates an infection that needs oxygen, the techie term is aerobic.
The best solution is to bottle it, leaving the scum undisturbed and about 3-4cm of wort behind. Hopefully this will leave the wort behind that is changed by the infection, meaning minimal off flavour transfer to your beer. Quite a few people have bottled and consumed beer from fermenters showing similar grey growths.
There is no such thing as a protective layer of carbon dioxide. The air above the wort is a gas, and gas molecules mix freely, they do not settle into a protective blanket.
What does happen during fermentation, there is so much carbon dioxide produced, it flushes the headspace leaving mainly carbon dioxide behind.
20gms of dextrose will not produce enough carbon dioxide to flush the headspace. It will not kill off the infection. It will not prevent reinfection. The beer is infected. The only way to kill it off is by heat treating to 125 deg C for 10 minutes. This is not practicable for us homebrewers and would also change the beer flavour. Your only course is to hope it is an infection that needs oxygen to thrive, limit the oxygen, bottle it and cross your fingers it is still drinkable.
Your brew is infected, no amount of scooping or dextrose will change this.
The whole brew (not just the top few cm) will contain spores that can and will reproduce the same infection again. Make sure that your equipment recieves a very good cleaning before the next brew.