Appeal to the masses?!? I estimate that around 1000 cars per year are converted into EVs... In the entire world. Appealing to the masses doesn't even factor into our design process. If it did, we wouldn't being making stuff for this market in the first place!
I've previously mentioned that I am not a big fan of "all in one" boxes unless there is a very compelling benefit to doing so. In the case of the dc/dc and charger, the main benefit is that it could allow kicking up the output power of the dc/dc for "free" because the money that would be spent on the output filter inductor and enclosure could be redirected to the actual power conversion components.
Any other benefits - such as reducing the amount of wiring - are more subjective, since that may or may not be relevant to a particular installation. For example, the charger should be mounted as close to the battery pack as possible while the dc/dc should be mounted as close to the 12V battery as possible, and those two things may be on opposite ends of the car.
So I am wavering on this one...
...If you want serious power from the main battery pack you can use a VF drive....
This suggestion is creative, but bordering on irresponsible. Not every AC electronic device will appreciate getting hammered with a PWM approximation of a sine wave without first being LC filtered and most VFDs do not like to see unbalanced currents on each of the three phase legs. All in all, a recipe for disaster.
You plan to use bipolar transistors, mosfets, or IGBT's for the switching of the DC? Also are you going to use RC or RCD snubbers because I get tired of seeing such a cheap out way of taking care of back EMF from the transformer input side rather than use half bridge synchronous rectification to have a path for the back EMF. I say use half-bridge synchronous rectification. You are the quality over quantity type after all.
I use whichever device is most appropriate for the job!?! That said, I haven't used a bipolar transistor in a switcher since the early 90s. 99% of the time I use MOSFETs, and only very recently have I considered IGBTs, as their higher current vs. die area is rarely worth the more complicated gate drive circuit required at power levels below, say, 1-2kW.
I don't get your distaste for passive snubbers. They are a reliable way of both absorbing the energy stored in the leakage and stray inductances in the circuit, as well as damping the Q of the inevitable resonances between switch output capacitance and said inductance. Most means of recycling this energy - by using a quasi-resonant switch, for example - result in a less rugged design as a result of a disturbingly high tendency to fail at light or no load.
Finally, you do not plan to use a TL431 voltage reference as a bastardized OP amp in which they were really never meant to be used this way as so many manufacturers like to use them....
It is true that using a TL431 to drive an optocoupler for the feedback loop in a switcher is a bit removed from it's intended purpose as a variable shunt regulator, but it is almost always the opto itself that limits dynamic range and transient response, not the TL431! Dr. Ridley wrote an excellent article on using the TL431 in a power supply some years ago which I highly recommend reading.