The rear wheel steering makes the vehicle drive strangely so even if it is not a problem for its driver it is going to move in a manner that the other drivers are not going to expect. This makes it more dangerous.

I would not want to drive that windshield into a sunset, rainstorm, or oncoming headlights. Polycarbonate is very easy to scratch, but very hard to polish.

In the cut away picture of plastic body I noticed the floor cut out. Is that so the occupants can drop their legs down and run like the Flintstones car? What is the top speed of this car? It takes speed and weight for regenerative braking to be significantly effective.

Wait, that sounds vaguely familiar... 40 miles on electricity with a gas backup engine. Didn't a big company in Detroit come up with that idea about 10 years ago? Oh right, it's called the Volt.

Please explain how stability will be achieved with inherently unstable rear-wheel steering. One would think that any over-input to steer the real wheel could easily become hugely magnified, resulting in catastrophic loss of control. Clearly this challenge has been dealt with. How?

While cool looking it has a few things wrong.
First thinking a diesel in this size is eff isn't true and for much less money, weight with better eff get a racing gocart motor with 14-1 compression ratio set up for ethanol that is.
You could get 15 hp as eff as a 7hp diesel at 50% of the weight.
Next ultracaps are an expensive overweight scam. This alone shows they are not real smart in EV drives. Use the money, weight for more battery.
Why still doing metal framing when well done composite body/chassis is much lighter, stronger, cheaper and more space eff too? They have already done the hard work in the shape. The rest is just lay up schedule in a mold made from the prototype, simple, cheap, stronger than steel.
The windshield angle presents so many problems from distortion, heat gain, limited ability to see, etc and not needed for good aero.
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the thrust ssc used rear wheel steering , they tested it on a purpose built mini chassis and it' completely safe and stable .

Kind of cool, but totally impractical. One would need to be a dwarf just to fit in it.

It it can win the X-Prize & sell under $45K I would buy it.

No I like this project and I disagree with some of the comments here.
The rear wheel steering is fine for a docile-use vehicle like this. Every boat, aircraft and forklift use it after all! The 3 wheel arrangement obviously has negative implications for stability though. Again, it's for docile driving and this a tech demonstrator.
A go-cart engine is efficient when being flogged but even with changes to the oiling and fueling, they're not really suitable for constant load applications. A small non-turbo diesel is harder than a large diesel to make truly efficient in SFC terms, that's true, but it's the right type of engine so it's better to start there and tweak. I'm not sure what they're saying about ethanol though - pure ethanol is useless for diesels and messing around with blends of it seems beyond the scope of what they're doing here.
I agree with the decision to go with a metal frame - if composites were worth it for limited runs, they'd be in every major race car competition. Composites are the future but if I was going to drive this thing across the US today, I'd prefer an old fashioned frame. Also, composites in aircraft haven't taken over like everyone predicted. The economies of production and the potential for super strength and awesome shaping of components hasn't translated properly from theory to practice. That might be a production volume thing, I'm not sure.
I don't know what the return on weight/cost is but ultra caps have great potential for the speedy storage and then dispensing of small amounts of energy. I'm thinking more about stop start traffic than long mountain descents though. As an aside, I would have thought a a small gas bottle hooked up to a device that acts as both air-compressing brakes and an air-powered motor would be more efficient though. Might also be beyond the scope of this project :-)