The term "runaway warming" or "runaway GHE" has a specific definition in climate science. For Earth, it would require about 80 additional W/m² of solar irradiance, bringing it to the K-I limit, boiling off all of the oceans while decoupling the Planck response, resulting in a surface temperature near that of Venus. No, you have not constantly heard this.

GCM projections have very successfully simulated a variety of future and past climate conditions.

Consensus reflects the state of knowledge of the scientific community. It is very much a part of science.

Speaking of which, again, the consensus "best" estimate of ECS is 3ºC. (1.8 for TCR). Feedbacks supporting those values are clearly evident in simple calculations comparing forcing and temperature increase in the record. And, of course, glacials and interglacials in paleoclimate would not have been possible without those feedbacks.

Historically, human civilization has likely not seen global temperature as high as now.