It's all in that first 0.5 ratio. Basically, that ratio says that if I have a 10 ton aircraft, 5 tons is for the payload, airframe, engines, miscellaneous structure, and takeoff/landing batteries while 5 tons is for the cruise batteries. That 0.5 ratio is probably very optimistic for a real-world vehicle, but not crazily so. Burt Rutan's Voyager and the Virgin Globalflyer got much more ambitious figures, like 0.75 and 0.83, respectively (in this case referring to fuel mass instead of battery mass).

You refer to L/D ratio and lack of wing loading as if it's the same thing but it isn't. High performance gliders are generally ballasted (sometimes about half the weight is ballast) in order to increase the wing loading and increase flight speed. Our aircraft would have a 300km/h cruise speed, and so will similarly have a fairly high wing loading, even if it does need long, high aspect-ratio wings. This is helped by the fact that the sort of high performance lithium-ion batteries that you'd be likely to use (i.e. like Tesla uses) would be a good twice as dense as fuel. This increases your effective wing loading for the same outer mold line, allowing higher speed efficient cruise at the same altitude.

As far as battery and support system being a central failure point:

One interesting thing with electric propulsion is it's fairly easy to split the batteries and subsystems up /as well/ as the motors. So each motor pod could have its own small battery nearby for (possibly emergency) take-off and landing (this could also help reduce cabling mass, especially for the high currents you're likely to need for the vertical takeoff and landing portions). The cruise batteries could remain centralized, since you'll need a lot more of them and mass efficiency will be critical (and you could rely on your very good glide ratio).