I have no problem with most of what you say, Chris, although the writing itself is never so cut-and-dried. Here are examples: WB does give the arm with palm turned down and stroke as a form of “Elle” (cubit) and “Ellenstab” (cubit rod, but also immediately above simply as :der Arm,” the arm. WB II, 120. It thus appears to be a variant with meaning depending on context. In fact on cards 574a and 574b of the Belegstellen, there are examples from the Pyramid texts with palm down and stroke where the meaning can only be arm. That this is not a transcription error is indicated by the use of
so (sic), in each case.
Moreover, other cards under mH in the Belegstellen indicate that at all periods variant writings were used, both palm up and palm down for the word when it
does mean cubit. One card (I can’t seem to link to it) states” :belegt seit PT.
Das zeichen wechselt anscheinend zwischen [D 36, D36a, D 42].” (Attested since the PTs. The glyph apparently varies among D36, 36a, 42). Italics mine.
As for zA, the word for "son" is almost always written as pintailed duck (G39) + stroke, and I am sure you're aware of this usage. See WB III, 408 et seq.
Lee