I don't know much about it's use for nasal congestion, but I would try it.
My parents, from Germany, didn't cook with garlic. When I mentioned to my father, however, that I was eating it together with my son to hopefully cure him, he spoke to me about his father (my grandfather) who was a soldier in World War One. Infection was a huge problem at that time. He said that all the German soldiers would carry a few cloves of garlic with them and were instructed to crush the garlic in the event of being wounded, and then rub the mash onto and into the wound. It would act as an antiseptic to prevent infection, and proved to be quite effective. My grandfather was shot in the leg, followed instructions and recovered.
Years later, in the early thirties, that same grandfather cut his hand working on his farm, but continued to work and finish the day. His hand got infected; my father, a policeman at the time and familiar with the town physicians, begged him to see a doctor, but he wouldn't do it; he didn't trust doctors. My father said the hand looked terrible, so bad that he couldn't look at it (and that's saying a lot about how bad that hand must have been). But my grandfather applied garlic to the hand in the day, and in the evening he exposed it to maggots. My father was certain his stubborn father would lose his hand, but it recovered and got better.
Truly a wonderful medicine . . .