... and when plants arrived did they not take care of a much higher concentration of CO2 gases than is present today?
they did indeed - but how many millions of years do you think that took ?
Millions, not overnight that's for sure. Starting with one plant, I'll say it takes time.
Would not CO2 level calculations have to take into account the loss of vegetation? Continents drift, deserts and ice caps form, land erodes and many other natural occurrences affect growing space. If growing space is available and mankind has destroyed it thus reducing the capacity of vegetation to remove CO2, that would include oceans as well, then how can that not affect CO2 levels?