Since there is no way currently of removing CO2, and we don't even know how that technology would work, there is no fair tax amount. Any amount you set will just be a robbery. Might as well make it a cent per ton, or a gorillion dollars per ton.
The money that these things cost to the public health service is easily quantified. I expect the taxes to reflect that.
>making low CO2 alternatives much more attractive
If there were "low CO2 alternatives" you wouldn't have to tax stuff, you could just force people to use them, and no one would really complain. Like it happened with CFCs. But taking a train instead of driving your car is not an alternative, it's rubbish.
Bikes aren't an alternative because instead of adding bike infrastructure that costs 10% of car infrastructure, the freaking 6 lane stroad needs a 7th lane, because that's going to solve traffic!
Walking isn't an alternative because US cities and suburbs are largely devoid of trees and any sort of protection from the sun or rain along their sidewalks, if they even have sidewalks to begin with. Oh, trees also make for a nice view and protect pedestrians from bad drivers, instead of lawns everywhere (!!!!). Plus trees and some bushes contain the crazy, deafening, ear damaging levels of noise cars make at speeds higher than about 70kmph.
Worse, it is common to actually subsidise fossil fuels.
There are other reasons to plant trees (well, plant plants in general), notably around rehabilitating land that is on the verge of desertification or the like.
Most of the worlds large forests are no more, many European countries are stripped of their old forests, so replanting those are required anyway, for other environmental concerns. They could on a permanent basis capture some percentage of the CO2. It is permanent if you view it as a forest and not individual tree.
I think that's basically correct on the scale that's needed. I don't even think turning trees into building materials is enough.
Planting a lot of trees and letting forests grow has the advantage that it's obviously possible with current tech, while the forest is expanding in mass that CO2 is being scrubbed, and it's conspicuously visible to people (garnering feelings of goodwill and participation), but I don't believe the relatively small amount of incremental forest that we could reasonably create will make a meaningful change on a 100-year basis.