What's wrong with my mathematical interpretation of CO₂?

No your maths are correct. The interpretations are as follows

• We assume that CO₂ never previously reached these levels in previous cycles. I don't think ice core data would necessarily capture higher levels once temperatures reach a certain level

• We cannot account for water vapour. One of the complaints about the Law Dome samples was that recent times had double layers. This could be a water vapour issue. Once the temperatures rise to a certain level, we will get increased water vapour which as you pointed out will reflect sunlight. If less sunlight gets through, less can be trapped by CO₂ ... creating a downward pressure on temperatures.

I will never argue that there isn't climate change ... might as well admit it is just climate. By my speculation, we have just about reached the pinnacle of temperature rise and might be just about ready to start down the 100,000 year cycle toward an ice age.

Whether cooling is a nice thing or not is a question of what people prefer to die of. The little ice age was not exactly pleasant.

The Little Ice Age describe wild climate swings with extremely cold winters followed by very warm summers, and cold wet years followed by cold dry years ... Bad weather resulted in the Great Famine of 1315-1317 which decimated Europe causing extreme levels of crime, disease, mass death, cannibalism and infanticide. What caused the tragic climate changes of the Little Ice Age? Some scientists suggest lower solar output associated with periods of fewer sunspots. Increasing solar output then reversed the cooling and warmed the 20th century world. As solar output is now falling to the lows of the Little Ice Age, a natural experiment is now in progress testing that solar theory. However other scientists suggest it was rising CO2 that delivered the world from the Little Ice Age.
Increasing CO2 also has a beneficial fertilization effect that is greening the earth. The 20th century warming, whether natural or driven by rising CO2 concentrations, has lengthened the growing season. Famines are being eliminated. Tree-lines stopped retreating and trees are now reclaiming territory lost over the past 500 years.
Source

The little ice age killed a lot of people. In a period of about 200 years the world's population declined from 450 million to 374 million. When temperatures are hot crops will grow (with sufficient water). The same cannot be said of when they are cold.

Info pulled by google searches:

• Canada produced 716 megatonnes of carbon dioxide equivalent in 2017.
• Algae can absorb over 2 million tons of CO2 a year per acre.

To absorb the entire CO2 production by Canadians (in 2017) would be less than 1000 acres. (395 ... I have added a fudge factor because Canada wouldn't necessarily have the same efficiency of absorption as other countries).

If governments were really concerned about CO2, we would have a proliferation of Algae farms. The reason we see so much about carbon and climate change is that there are people making money just talking about it and I don't think it would make a positive difference anyway.