I'm sorry but I think you're missing a lot in your assumptions.
For example, there are only rough estimates on how much methane is trapped in the permafrost. The first estimate I saw by Natalia Shakhova et al was 1400gt over a decade ago (so about 45 years of human emissions at the current rate). I found this recent MIT article which mentions 1500gt [1]. Let's not forget methane has a 25x more heat trapping capacity than CO2. Do the math on that. Also the permafrost is just one of many feedbacks.
Another point you're not considering is that the capacity of the biosphere to absorb carbon is being reduced. For example, the Amazon used to be considered a carbon sink but it is becoming a carbon source [2].