1) Keep 1.8V <= 3.3V as the power rises. If you look at our reference designs you will notice that the 3.3V and 1.8V come up at the same time
2) I am not sure what the exact question you are trying to ask. Can you re-state the question please.
3) Unused DAC pins can be left open. Check the data sheet 'Unused Pins' paragraph.
