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Much less than 100% of electricity is... still more efficient than ICE? I don't see how that changes things.

US electricity averages 384g CO2/kWh, so EVs emit 100-130g/mile, equivalent to 90-110mpg. And that improves year on year, while an ICE is at best as efficient as when it was produced

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while a very important aspect (and you're right that EV are more and more better on that), that isn't strictly thermodynamic efficiency you're talking about. For example, natural gas is a major fuel for electricity generation and, even at the same efficiency, it produces less CO2/KWh than say gasoline.

Methane (natural gas) is a powerful greenhouse gas, much more insulating that CO2. Leakage of methane during production and transport will have a huge impact over the short term (next 50 years). It is actually way worse than gasoline or coal for that reason.

By the time all the CH4 we are dumping into the atmosphere has broken down (half life ~12 years) we will have pushed global temperatures much higher.

Efficiency, output per input, is only relevant for similar inputs. An ICE being more efficient means proportionally lower costs and emissions, but like you said, an electric drivetrain being 90% efficient is less informative than a well-to-wheel comparison.

Similarly, I think it's unhelpful to just expand it to primary energy. What's the efficiency of a wind turbine? In 50 countries already, a 100% efficient ICE would emit more than an EV! The reason for efficiency here was costs and emissions, so compare those.

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