Cretaceous oxygen levels were at times comparable to those today, and possibly lower. In any case dinosaurs probably had very efficient lungs, as birds do. Unfortunately there's no way to get DNA that old.
http://levenspiel.com/wp-content/uploads/2016/02/DinosaurW.p...
Rather surprisingly, I just found that different oxygen fractions do cause different atmospheric pressures, by altering the density of air, but it's a 15% to 25% difference, not 200%. Maybe it's possible to use this to resolve tensions in estimates of how much oxygen there was. See https://www.science.org/doi/10.1126/science.1260670
Like a lot of island species, they are smaller than their continental counterparts and when relocated to the continent, they gain on average about 100 pounds and grow thicker fur.
https://www.upi.com/Odd_News/2004/12/16/Catalina-buffalo-ret...
Humans who live at altitude for any length of time can see their lung capacity increase. Human sub populations that have lived at altitude for generations also have other adaptations, such as thicker blood, more small blood vessels, etc.
That's in the current world for just a couple of species.
As far as I understand reality, dinosaur physiology is not adapted or suitable to current conditions on planet earth, including but probably not limited to the way their lungs work and differences in atmosphere.
This is probably a good time to agree to disagree about the rest of it.