Many "past Earths <that> had lots MORE methane & CO2" than ours were metastable in states that we would find impossible to live in. I would not want to shift our climate to a state in any way similar to those!

When you make a change to the environment, it adjusts and finds a new equilibrium. The main questions are how many oscillations you have to go through before you find the new "stable" state, how severe are the oscillations, and what the new "stable" state will be like.

21st century modeling is an awesome tool. But no one should (and few do) trust a model till its product can be compared with actual observations. When the model is supposed to predict climate, a century in the future, specifically taking into account something that has never happened before (continuing release of massive amounts of carbon into the air), it's a pretty tough task.

However, these models are the best we can do. And I appreciate that we will probably disagree on this as well, but most researchers and modelers are honest scientists, deliberately trying to remove their beliefs from their results. That they sometimes fail just means they are human.