I had an earlier K12 auto which had a similar problem from when i bought it from a dealer s/h-I went through all the same kind of stuff that you have-it turned out to be a faulty thermostat causing cold running=more fuel injected = bad economy, would barely achieve 30 mpg, changed thermostat, problem went away, economy back up in the mid 40's- result! If the temp gauge is reading in the lower part of the range ie less than half scale it's running cold.

All sorted, was a £12 part.

Cost £70 all in, with new coolant and labor costs included.

Straight back up to 350-400 miles to the tank.

Thanks!