It looks like we were as high as 1000ppm less than 100 million years ago.

Yep, if we return to the hothouse of the mid cretaceous, we'll all be pretty completely fucked.

Our own ancestor was probably a placental mammal already. Tyrannosaurus Rex and Triceratops hadn't appeared yet. But any infrastructure built by Pleurocoelus would be well suited to the new sea level.

Unfortunately there isn't any, and the most valuable part of our infrastructure would be under water.

This graph sure as fuck makes it look like we are coming out of a cooling period

The broken axis is misleading. The SCRIPPS graph I link to above is a clearer view of the current warming on top of the glaciation cycles of the Quaternary. We're warming from the top of a warm period.

which may explain the entirely of the C02 increases

The CO2 emissions from fossil fuel combustion already entirely explain them.

Just because C02 is a greenhouse gas, and we have observed it increasing, does not mean that C02 has increased the earth's temperature

Yes it does. That's what an increased greenhouse effect means.

There are confounding factors, such as other gases having a much much larger greenhouse effect.

Methane has an impact too, on the few to several centuries scale. But CO2 is responsible for 74% or 91% of the long lived greenhouse gas warming, depending on whether you count or discount the overlaps with other greenhouse gasses.

If only you had some data to support this assertion.

https://gml.noaa.gov/ccgg/carbontracker/

What if temperature increases caused C02 levels in the atmosphere to increase?

Then the CO2 would need to come from somewhere. And it's not. The oceans are absorbing CO2.

Edit: How am I supposed to conclude that there's a causal relationship from this graph?

Don't use that graph. The CO2 levels are from an older version of geocarb, and the temperature levels are from and even more ancient estimate that has been thoroughly superseded. It was from a time when it was thought that the earth just oscillated between "warm" and "cold". It's not true, and is not calibrated to the same timescale. You would need to be trying to mislead your audience to plot those two together on the same chart, because we had much better temperature reconstructions even at early versions of geocarb.

CO2 has warmed the earth at between 1.5°C and 6.3°C per doubling throughout the last 420 million years at least.

Are you starting from the assumption that all C02 increases since 1750 are because of fossil fuels?

No I'm comparing the atmospheric increases in CO2 since 1750 to the CO2 emissions from fossil fuels since 1750. The emissions from fossil fuels are 154% of the increase in the atmosphere.