Is it intentional that the majority of the squalene in the human body is secreted in sebum by sebaceous glands in the skin, our natural defensive barrier to environmental insults?

You are seeing intuitively and reasoning accurately, the true situation. We designed a protective system for the skin, being the largest organ and the interface with the environment encapsulating your physical body and shielding it from many outside adversarial influences, with a protective fluid designed to coat the skin and persist. We imparted it with antiviral activity deliberately, so there would be an on-site defensive mechanism chemically, as a further precaution to assist human safety and well-being. Just like everything else throughout the phyla containing beneficial medicinal properties, this is all planned and orchestrated to provide a basic foundation for a successful physical realm where many species can exist and flourish even though competition is built in as well. We saw the need for defensive measures, given that organisms also have combative instincts and weaponry that can pose problems if things get out of balance. So to provide a basic foundation for stability there must be resistance to an immediate overtake by things like microbes that could overwhelm higher organisms and make prolonged life impossible. We need to give everything a fighting chance, so to speak, and you will see this reflected in the many examples of phytonutrients containing antimicrobial activities as well as anti-inflammatory substances that counterbalance aggressive immune system defenses and on and on through many layers of cause and effect exhibited by organisms sharing the physical environment of the biosphere.