Regarding your question, how could Visnu appear from the nostril of Brahma? The answer is that Visnu being all pervading, He can appear from anyplace. He appeared as Nrsimhadeva from the pillar. So, one may question how Visnu may appear from pillar, but actually He appeared. He is all powerful with inconceivable potency, and therefore He can appear Himself from anywhere He likes. When Lord appeared as a small hog from the nostril of Brahma He began to expand Himself more and more and gradually He became a gigantic boar. So these are inconceivable energy of the Supreme Lord. It does not, however, mean that Brahma’s nostril is the birthplace of Visnu. The sun rises from the eastern horizon. That does not mean that eastern horizon is the birthplace of the sun. I hope you will understand the transcendental appearance and disappearance of the transcendental Personality of Godhead as such. In the Bhagavad-gita therefore it is stated that anyone who understands the transcendental position of the appearance, disappearance, and activities of Krsna becomes liberated immediately after quitting the present body.

(From a letter by Srila Prabhupada — Unknown Place Unknown Date)