"Would it have been possible for the dinosaurs to live during a time when there still existed huge quantities of CO2 in the atmosphere?"

Nope.
I would guess top threshold would be around 10% CO2. Currently CO2 levels in outside air is around 1/2 of one percent. If air had around 5 % CO2 then it would start affecting some people- 10% would probably impair most people & higher levels would start killing some people. I'm not sure about cold blooded animals (and there is some doubt about dinosaurs being cold blooded); it's possible cold blooded creatures could tolerate higher levels of CO2. One example of high CO2 levels is holding your breath- the problem with holding your breath is not lack of oxygen but rather the increasing levels of CO2 in your lungs. Another way of looking at it is the fish (cold blood creatures) operate in an environment which has high levels of CO2.
But the amount CO2 you are talking about would definitely kill all creatures- it would even put out any fire:)
Maybe if you increase the percent of Oxygen- if you doubled O2 to say 40% you might get away with higher levels of CO2- but still no where near the ballpark you are talking about.
One way to increase the density of atmosphere, might be to *somehow* increase the humidity in the higher atmosphere- it's pretty dry at the present- don't know how you could do this if it's at all possible.