The requirement to run on premium has nothing to do with the type of injection used, it's down to the relatively high specific power of the 1.4 engine. Using DI to produce the same power from the same engine displacement would have made it more likely, not less, to require higher-octane fuel, because using DI increases combustion temperature (and that is why DI engines produce more NOx). One of the most common misconceptions about premium gasoline is that it gives better mpg: it doesn't. Higher octane rating means the fuel can be used in a hotter engine without pre-igniting: nothing less, nothing more (actually, in terms of calorific content, high-octane fuel normally has less energy in it that regular); it's more correct to say that hotter-running, higher-output engines require higher-octane fuel.A car that got 40 mpg highway would have come due to the strings attached by the task force. It didn't have to come at the time it did, run on premium, or be on a widened C-Evo CUSW. It had to run on premium as long as the car was 3200 lb and they didn't give the engine direct injection. If they just tacked a trunk on Giulietta and made it in the US with a Dodge nose that would have saved 200 lbs. If they made it a sedan on SCCS SUSW they could have saved another 200 lb (2800 lb). An economy car running on premium was doomed to failure in the US. Unlike Europe the point is lower operating cost, not avoiding multiple CO2 taxes.
I agree with you about Dart being too big, although as someone who owns a Giulietta, I would add that it would have needed a wheelbase stretch of 2-3 inches in addition to the trunk in order to reach the size of a typical compact sedan (e.g. Corolla). The extra width was not required - Giulietta is actually slightly wider than a Corolla both inside and outside. (But when you see Dart as a rolling prototype for KL and 200, making it that big makes more sense).
It's also not true to say Europeans buy "economy" cars to avoid paying environmental taxes: low fuel consumption is still the overriding goal for buyers, all that the CO2-based taxation schemes do is make this clearer to buyers, because it is, in effect, a tax on vehicles with high fuel consumption. Burning a litre of petrol produces the same amount of CO2 regardless of what vehicle it's being burned in. The differences in emission are down to how much of that fuel gets burned. The banding in most systems does occasionally magnify some very marginal differences (e.g., a 1 gram difference that puts you into a harsher tax band), but overall, by buying a "low CO2" car you will save much more in fuel than in annual taxation.