I am a bit confused...but here you go. This is what I know.
hereabout and whereabout are initial letter contractions.
According to 10.7.1 these types of contractions can be used wherever the letters they represent occur (unless another rule prohibits that). I can't find any rule that prohibits either of these being used in hereabout or whereabout.
The rule for shortforms says what you noted in the first post above (I'm not going to repost it here).
So I then go to Appendix 1...which deals with shortforms. Look at the list regarding the shortform word 'about' (page 229 of the code). Both hereabout and whereabout are on that list...so both are acceptable.
Now we add the 's'. If you look at the paragraph at the very beginning of Appendix 1 (under "The list") it says that when an 's' or apostrophe 's' is added to any word on the list, use the shortform with the following exceptions...I realize that abouts is one of those exceptions...but it is not the word hereabouts or the word whereabouts...it is the word abouts all by itself (and that's probably because the contractions abs could mean abs [a different word] rather than abouts).
So hereabouts would be (here)(about)s and whereabouts would be (where)(about)s. Since hereabouts is shown as an approved word on page 138, my logic is sound...and would apply to whereabouts in the same manner.
Does this help?