OK - was just on another newsgroup, where someone said if the circuit
is existing, I basically just abide by the rules as they existed when
the circuit was run and don't need to run a new 20A circuit to the
bath. Can someone confirm this? In other words, basically all my
problems go away. So then I would have:

Circuit #1 (existing 15A)
1 - circuit comes from panel to upstairs master bath outlet, where I
would add a GFI outlet first in the chain
2 - then over to outlet #2 in upstairs master bath
3 - then over to upstairs 2nd bathroom outlet
4 - then downstairs to 1/2 outlet
5 - then outside to the outdoor outlet

Circuit #2 (existing 15A)
1 - circuit comes from panel to upstairs master bedroom receptacles
2 - then into upstairs master bathroom for lights/switches
3 - then into upstairs closet for light
4 - then into upstairs 2nd bathroom for lights

Pretty much the only thing I'd have to do is swap out an outlet, and
insert another one in the line.