maybe it's not a voltage divider at all, and that is simply a bypass capacitor to reduce noise on the voltage reference line.
Yes that's it.
It wants an imaginary number and the radian frequency of the input voltage.
In electronics an imaginary number is one that is on the Y axis and represents a phase shift of 90 degrees.
Radian frequency is simply 2pi f
With a single dac, I'm not sure what the point of the buffer being there is.
So you can load it in any time before the next sample is due but not output that sample until a clock pulse.
I guess all I need to know now is how the DAC can output 2x the input voltage.
It can only do this if the reference voltage is at least half the supply voltage to the chip. This also applies if the chip is being supplied by split power rails. That is a positive and negative voltage, then if the reference voltage is 4 volts then the output can go from -4v to +4V.