>I’m not an expert on this, but I thought CO2 is well mixed within the atmosphere, and I think surface temperature (or tropospheric temperature at whatever height) is certainly not “well-mixed” across the globe.

I am not an expert on this either. I don't know either how well CO2 mixes over the globe. But I could imagine that
there are even "vertical" differences, depending on the different airspheres and it seems that there are
"horizontal" (i.e. regional) variations, Wikipedia has this animation about mid-tropospheric concentrations.

In particular if you look at the CO2 curve discussed above then there seem to be seasonal oscillations, so alone by this curve one can tell - that the CO2 can't be too well mixed.

Similarily it would be interesting to see how localized the methane concentrations are. The Wikipedia animation is especially interesting in that
respect. For example it looks (the globe is turning quite fast) as if there are strong CO2 concentrations over Siberia, this
may be partially due to a thawing permafrost which releases methane and which in the turn turns into CO2. However these concentrations could also be due to "winds" from Europe. Like the big concentrations over the Atlantic next to the US east coast seem more likely to be due to the industry emissions of the east coast, then like stemming from gas hydrates.