It seems that some of the information you have doesn't apply to the SAP-2.
Earlier in the book, it said that to execute a decrement instruction, you must load the value you want to decrement in A, subtract 1, and then load it back into the designated register.
The Simple As Possible SAP-2 computer has dedicated decrement instructions.
Section 11-4 "...
The repeated-addition algorithm you describe is not the usual way. Instead, think of long multiplication, but in base 2 instead of base 10. The shift instructions available in most CPUs are useful here.
Here's how you might do it on a 6502:
; shift result up by one place