Answer from cs61c-cj (Peter Lau 16539384) for Question 2
The system would not work as well if we placed the SPN to the right of the 2PN because of how spatial locality works.  Programs will access memory as the memory addresses are continguous.  However, if the SPN bits are to the right of the 2PN bits, the "adjacent" virtual memory addresses may map to different SPN page tables which will map to a different page.  Virtual memory uses pages (or chunks) of memory in the first place to take advantage of spatial locality in programs and in the way that disk accesses take place.  Switching the bit patterns will invalidate these gains.