Answer from cs61c-ej (Leon Wang 16247444) for Question 1
Virtual address add a layer of addressing over the physical addresses and the disk addresses.  Virtual addresses map to both physical addresses and disk addresses but each virtual address only has one mapping.  By using virtual addresses, you have a continuum of addresses that include the physical addresses and the disk addresses even though RAM and disk are separate entities.

The whole point of mapping virtual address to physical and disk addresses is that it makes the amount of memory you can access look bigger.

You can get a PA from a VA.  It's just a mapping.