Speciation isn't one and done, it's a shift in the average of a whole population, so this would be part of it but definitely not the norm

Evolution is a complicated thing and hybridisation shouldn't be brushed off. It's quite common in more closely related animals, which leads to different results than random mutation. It's likely what you said was taught to you at one time but evolution is one of those things that's always overly simplified and it annoys most biologists I know.

Yes it reaches an equilibrium with the atmosphere, same way a sodastream works

the test was done in serum, unless you're storing bodily fluids it's not really the same.