VCXO Control Range Above 0V
Yes, your interpretation is correct. 

The simplest, and possibly most precise, solution is to not bias the DAC but to just ignore the 10% (or 20% counting the top end) of resolution that is lost. DAC resolutions go up in increments of 2:1 so chances are the 10% or 20% is insignificant.  

What is the point? Just speculation, it's easier inside the oscillator. Also it's good to avoid the extremes of the power supply rails because it's not always possible to get accurate analog circuitry that operates very close to the rails, so keeping 300mV away from the positive rail and from ground makes sense.