Am I correct in the following points?...
P1. Regardless of the source of CO2 (natural or anthropogenic), increases in atmospheric CO2 drives increases in global temperature.
P2. As per the paleoclimate data, CO2 levels rise following increases in temperature.
Without considering any other factors, P1 and P2 should lead to a geometric progression (feedback cycle) of increasing temperatures and CO2 levels.
If the above points are correct, some questions....
Q1. Regardless of the source of CO2, at what point do CO2 levels have to reach to engage this feedback cycle so that it is irrevocable (self-maintaining)? For lack of a better term, I'll call this the Point of No Return. (PoNR).
Q2. Have we reached the PoNR?
Q3. What mechanism(s) breaks this feedback cycle so that temperatures and CO2 stop rising?
Q4. Related to Q3, what mechanism(s) causes CO2 levels to drop below the PoNR where they cannot restart the cycle?
Trying to answer your points as independent items, but without meaning the chain they form is right:
P1 - Yes, CO2 is a greenhouse gas, strong, but its effect is partially saturated, so you need a lot of it to raise global temperatures and the effect is not experienced immediately because of delays in the global system, like the time needed to adjust ocean heat content. Current accumulated anthropogenic emissions, and those in the foreseeable future, do amount to "a lot of it".
P2 - Not always, but it's very common to be that way, but not necessarily in the same proportion.
So Ps are good enough to allow the following:
Q1 - There's not necessarily a PoNR. The process always have depended on carbon reservoirs that may be ready to "burst" in case of sudden warming, like peat, permafrost and the ocean becoming a source if heated enough. Some may tempt you with sudden destabilization of methane clathrates, but that is very unlikely to become by itself the cause of an escalation of global temperatures though it remains to be an important co-factor.
Q2 - Certainly not. Such catastrophic changes only can come from huge sources of carbon and, unless there's some especial super volcano about to erupt, only humans burning coal during 100 years to generate five times the electricity we use today could cause such a thing.
There are positive feedbacks though. The permafrost contains more than a double of the carbon the atmosphere contains nowadays. But most of it is very deep and not ready to emerge in a couple of decades.
Q3 - Eruptions end, peat consumes, permafrost evolves into taiga or temperate forests, ocean inorganic carbon content equilibrates with the atmosphere's over it (of course, oversimplified)
Q4 - A lot, all of them slow: soil weathering currently absorbs something like 1 GT of carbon dioxide a year. Carbon in sea water through organic and inorganic processes continue to fall to the bottom or accumulate (would that amount to another 2 GT a year?). This process is jeopardized by the very same high levels of carbon dioxide, but it never stops.
So, for instance, if all the carbon inside the permafrost reached the atmosphere during a 50 years span -something near to impossible- some 3000 years of soil weathering should take care of it.
This is a rough summary just to give some answer to your questions. If you should ask, the largest carbon reservoir of them all is the sea water.
