Hi Julie. I'm glad you asked these questions.
Example 12-4 is a spatially arranged subtraction problem. No numeric indicators are used in spatially arranged problems where computation is to be done. [NC 25.3.1]
Example 12-2 is a spatially arranged fraction, not a problem for computation. It follows regular NI rules. The replacement numerals are preceded by a space and so a numeric indicator is required. [NC 3.3.1]
Regarding alignment in Example 12-2 (the fraction), replacement numbers are centered above or below what they replace. If exact centering is not possible (as in the numerator in that example) then the replacement number is placed to the left one cell, just like centering anything else in braille. I don't see this stated in the code book however.
Lindy