I kinda agree, lots of different formats in every direction, lots of dividing 1 by numbers to compare things.
One site lists Wh/mi, another Mi/KWh, manufacturer site only lists the range based on speed.
Then comparing it to figures for countries using metric distance, customary sized gallons for ICE, and L/100KM...It gets fiddly to make direct comparisons!

On the efficiency of generation, I guess it's open to the reader to apply their own modifier.
I'd be aiming to charge the car using private solar as much as possible which would drive it down.
National Grid emissions in the UK last year were about 217g/KWh on average. Even using grid the whole time, the emissions would be easily halved for me.

Edit: There is a suitably lengthy wikipedia page on MPGe. Having skimmed it, MPGe doesn't take into account upstream efficiency. While well-to-wheel gives a clearer picture, I can understand why for a simple metric MPGe does not. Especially since the primary function will be users gauging cost, and the electricity source should gradually improve over time.