Both as you mentioned. Charge circuitry for DCFC can be >200kW.

Motors, for instantaneous current, can easily exceed 100kW, some much much more than that.

Even assuming limitations to 100kW (which, would be very low for motor current), that's still 2000 amps at 48V. Remember, 100kW is ~134 hp.

Charging speed is directly related to the voltage of the pack. Even if your own vehicle had arm-thick cables to support high speed charging at 48v there is no quick charger in the world that could support it. You would be stuck in the bad old days of needing hours to recharge the battery on your EV.

No it's not, only in a practical sense. If you truly had 'arm thick cables', you could definitely charge a 48V battery just as fast. Practically speaking, though, you don't do this because every becomes so unmanageable that you can't build a charger, bus bars, etc, that would be able to match the charging speed.

The problem isn't the cables in your car, it is the cable between the DC fast charger's transformer and your car. They are already thermally limited, which is why you need higher voltages to support faster charging.

Like I said, this assumes you use ridiculous cabling and bus bars. You could design something that handles this, it would just be wildly impractically large and cost way too much money.

Also, the problem is definitely also the cables in your car. Moving to 48V would mean amperage would increase by 10-20x, which would mean cabling thickness would have to increase substantially.

I wouldn't see why not. A battery is internally a series-connection of lower voltage batteries.