"Venus would've lost next to zero CO2 - it's just too heavy."
I thought I recall seeing something about Venus losing atmosphere [obviously there is no end of discussion about the loss of water from Venus] but I google searched & didn't find anything. So we will assume that even from impactors or other events that Venus hasn't lost significant amounts of CO2. We will also assume Venus doesn't have measurable amounts of CO2 below it's surface. So that leaves Venus having 4.8 x 10^20 kg or 4.8 x 10^17 tonnes of CO2 for roughly the last 2 billion years.

"There's no way Earth ever outgassed 10,000 bar of CO2 in the last 2 billion years."

Earth mass of atmosphere: 5.1 x 10^18 kg or 5.1 x 10^15 tonnes so 10,000 bars would be about 5.1 x 10^19 tonnes.
It does seem a bit far fetched. It's about 100 times the CO2 on Venus.

I wonder what would happen if you added all of Earth's water [1.35 × 10^18 tonnes] to Venus.
It would probably acidify the ocean- though I don't know if you would end up with a ocean.

"The mass alone is the equivalent of ~40 kilometres of crustal rock over the whole surface. Falkowski's original quote is talking about biomass sequestration. There's only ~ 10 trillion tons of carbon in the biosphere. There haven't been that many (1.5 million) total burials of the biosphere in 2 billion years"

""Sea floor is not normally preserved for more than 200 million years," said Professor Rosing.

Most is destroyed at subduction zones, such as those found along the edge of the Pacific Ocean, where oceanic crust plunges under the buoyant and long-lived continental crust."
http://news.bbc.co.uk/2/hi/science/nature/6479289.stm

It seems that most carbon sequestration is going to happen in the ocean, and apparently, sea floors only survive for about 200 million years.

"I suspect the real figure is 15E+18 kilograms"

You could be right. It wouldn't first time there was a typo or something.