Metabolism is highly complex and there are many factors that affect fuel use. There are also big differences between all of the tissues in the body (fat vs liver vs muscle vs brain vs heart). Important factors in fuel use include uptake into tissues, enzyme activity and end product inhibition (i.e The Randle Cycle). Just because there is an enzyme pathway that could conceivably make a conversion does not mean it is operational in human metabolism. 

Ketone metabolism evolved to provide a simple and rapidly metabolized fuel source that would spare the use of carbs, protein and also they regulate their own production by slowing down fat release. There are very few steps involved in ketone breakdown compared to fat (beta oxidation) and glucose (glycolysis). Also there is no hormonal regulation or transport regulation of ketones into the cell or mitochondria. This means that, when present ketones are used in preference to other fuels and exert an inhibitory effect on the use of other fuels. 

In metabolism, ketones are not converted to carbohydrate.

Ketones can only be converted to acetyl CoA (ACoA) inside the mitochondria. 

ACoA has only 2 carbons. Each turn of the mitochondrial Krebs Cycle releases two carbons as CO2, so there is no mitochondrial pathway in mammals that can produce carbs/glucose from a 2 carbon donor like fat or ketones.  

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