ANSWER 1: It is based on the application and what you need to have turned on in order to maintain said application, what needs to be kept in a saved state, such as the DRAM, and what needs to be kept running in order to detect the activity that brings you out of the sate. In other words, it mostly depends on you.
ANSWER 3: Typical is 1.5W when in the UBoot mode. Again, depending on what you have running and connected. The USB host port supplies power to external devcies which will need to be added to the current numbers. The processor has voltage and frequency scaling that can be used to control the power, assuming these features are taken advantage of. You can cut the frequency to say 100MHZ and reduce this power greatly.
ANSWER 4: Yes. NO, you can also flash it into the NAND if you wish. Yes
ANSWER 5: Beagle I/O is 1.8V only. You can purchase a board such as the ZIPPY board that will provide access to the expansion connector serial signals. http://www.tincantools.com/