If plastics are eventually converted to CO2 via any process, it could be a pyrolitc process that returns the molecules back to petroleum distilates that will be burned, it could be by direct incineration, or biodegredation, we should not consider that plastic as being sequestered carbon.
Next, we should not consider sequestration per se as the objective. When we sequester CO2 on a permanent basis we are losing the oxygen from the atmosphere. Photosynthesis would give us back the oxygen from the CO2. If we burn up all the carbon on earth and sequester the CO2, we wil have removed most of our oxygen. Recall that earth has already diposed of by far the largest part of its oxygen when its silicates, metal oxides, and water were formed.
We likely have enough hydrocarbons and carbons that we could extract and burn to use up at least a quarter of our available oxygen. Volcanic combustions could use up more. We have no good handle on that, but burning hot metal does consume a lot of oxygen.
So our objective shoudl be to sequester carbon that has already released its oxygen. Short term sequestration, as in wood of a tree, plant matter in the soil, it delays ultimate release of the CO2. What about later, when it comes back into the air?
We can capture some of it again in plants. We can use the wood or plant matter to produce charcoal as a soil ammendment which further delays its return as CO2 and encourages soil to grow plants that recapture CO2.
When we Sequester CO2 in an oil well, some of the CO2 will come back out with the oil, even though a lot will remain deep in the earth, with its oxygen still chemically bound. We had to burn more fuel to create energy used to compress the CO2 to a liquid. Based on Australian measurement that is about 1/3 of the gross energy and half of the net energy. So, we are using up the natural resource 50% faster. Now the Australian process dumps the heat of compression into the ocean and it may be possible to recover a large part of that heat to power compression so that we might be able to improve our efficiency by a factor of 2 or even 3.
Cap and dividend by its nature is a gift to industries capped. I do not mean to the individual producer, but the industry as a whole. Even when our cap is on consumption of energy, and that puts at advantage any more efficient user of energy or particularly an industry that has fewer emissions because of their energy source, the overall effect is to force the industry to limit its output and thus reduce competition to sell. An industry that does nothing whatsoever to improve on emissions/product volume, when capped will have to reduce product volume. But miraculously that will raise selling prices, improve profit levels.
The single most effective way to reduce emissions globally is to reduce the amount of fossil fuel that can be extracted globally. This need not be done with respect to usage based on its emissions. If we reduce the extraction of fossil fuels by 1% per month, in 5 years we will have cut our total emissions in half.
This would not take into account sequestering in earth or deep in the seas. We could conceivably increase that to 70% with those sequestering efforts.
However, if we try to force emissions down while we continue to permit extraction of fossil fuels to continue unrestricted, its price will drop to maintain consumpiton of those fossil fuels. Note how sharply oil prices have fallen because consumption has dropped just below extraction levels. And any cap on emissions will force consumption down, prices down, and emissions right back up.