1) whether I can remap ANY digital pin to be the input/output to ANY peripheral (say SPI) input/output?
No. There are groups. If you have different modules of the same kind, they are usually assigned to different groups. So, it's more or less flexible, but not universal. It's better to assign everything before making boards.
2) on page 9 and 10 there is no SDO signal listed under the SPI column. I assume that is a typo? Or some other reason?
Muxes have one output and many inputs.
To route peripheral outputs, you have registers for every package pin. You write numbers to these registers. Each peripoheral output has its own number. The number determines which peripheral output will be selected for the given package pin. You do this with SDO and SCK (for master).
To route peripheral inputs, you have registers for every peripheral input. You write numbers to these registers. Each package pin has its own number. The number determines which package pin will be selected for the given peripheral input. You do this with SDI, SCK (for slave), and SS.
So, there are different tables - one for inputs and one for outputs. SDO will only be in the output table, SCK will be in both, SDI and SS will be in input tables.
Note that SCK is bidirectional, so it is found in both input and output tables.
3) finally and MOST IMPORTANT, I need to be sure I can remap the SPI pins as follows (2 different ways for 2 different applications:
PCB A PCB B OPTIONAL PCB C
SCK RA5 RC5 RB5
SDO RA6 RC6 RB6
SDI RA7 RC7 RB7
SS ANY ANY ANY
Are all of those combinations (or at least A and B) possible?
I'm sure you can look it up in the datasheet by yourself.