How are we actually alive?

[lestatdark]

Well, as for your fourth answer, there's scientific proof that Silica-based life could probably be sustained, due to Silica atoms have similar characteristics as Carbon atoms, for creating Sp bonds with hydrogen and other atoms, thus allowing for the appearance of Silicahydrates as well.

Also, not all life on Earth is solely Air-based, nor does it depends on direct sunlight or water as solvent to sustain it's life.

There are a myriad of bacteria that sustain themselves in Anaerobic fashion, be it with the consumption of CO2, NO2 (like the Nitrogen fixating Cyanobacteria) or even in some extreme cases, SO4 as electron acceptors in their mitochondrial respiration chain.
There are also Thermophiles, especially those who live underwater near Hydrothermal vents that sustain themselves exclusively in a process that's similar to Photosynthesis, in which they create glucose and aminoacids via a process called chemosynthesis, by converting heat, methane and sulfur emanating from those vents. 

While these examples are limited to bacteria, there's no indication on the contrary that elates us to the fact that complex (Eukaryotic cells) aren't able to be formed to be sustained in those conditions, but since the establishment of the overall conditions of the planet (78% N2, 21% O2 mixture in air; Water being the overall solvent and sunlight the prime energy source via photonic displacement), the majority of the common bacterial and eukaryotic cell precursors evolved in those conditions.

It's my personal belief that if conditions were different (not only the biological conditions I've just explained, but also electromagnetic forces, nuclear forces (if the electron spin orientation or Quark displacement were slightly different, we would have completely different atoms) or other physical integrity forces), we would still have life, just in a different fashion of that we know today. And we would be wondering if a life form such as ours would exist as well