It doesn't matter how quickly we put carbon in the atmosphere. It has a 200 year effect - taking 60 more years versus 20 is meaningless if you end up putting it all up there anyway. The only difference would be how soon we suffer the worst effects - it's too fast for adaptation in nature for most evolutionary or migratory processes either way. I suppose you could hope for a technological silver bullet but that's effort made while dealing with a much more costly carbon economy.

Also, we should really avoid the worst effects. Even the middling effects are pretty awful.

No, that's mostly on top of oil and coal extraction. (Also gas isn't big enough improvement on co2 emissions to the small extent it does displace coal or oil)

As the article pointed out, near the end, we'd also be potentially releasing far more methane, which could be worse than CO2.