So the numbering and header text are setup on/as a new (or should I say additional) pair of master pages, titled "Numbers" (L and R).
Whether I did this correctly or not, in one sense it does seem somehow I did join two sets of master pages: Two Normal's and two Number's to document pages 10-48. Document Pages 10-48 now appears exactly as intended. Not knowing any better at the time layering these two sets respectively, L and R, was what I intended to do. I don't understand, or should I be concerned, why Normal Right is "unused"; it appears as it should with properly formatted text throughout the book. Maybe I should have named the two new master pages "Header" L an R instead of "Numbers".
Touching on why the wacky numbers show up on document pages 3-9, (I thought all pages had to be accounted for, I was using the zero in "start" to represent none--empty). With neither header text frame showing then I didn't see that as a concern, the result is what I intended--blank headers. And initially it stayed that way though a couple of saves and closes.
Out of the blue, on document pages 3-9 the "Numbers" master pages (both header text frames) showed up where they were never applied. Studying your suggestions, I found the wacky pagination made sense, but why had it suddenly appeared where it was never applied?
The "show" box in Sections had checked itself. So I'll just un-check "show" again, click Apply and Okay. They were still there. I went back into Sections and the "show" box had checked itself again: un-checked again, Apply Okay again; still there again. It became a loop. Finally I just deleted the first pagination row in its entirety hoping the remaining row wouldn't jump up and populate document pages 1-9, let alone page 3. It didn't. Document pages 10-48 remain as intended.
However only the pagination disappeared (as intended). The other header text frame still remains with its text visible in document pages 3-9. Page > Apply Master Pages and re-entering the specified document page range 10,11-47,48 for "Numbers" L and R does nothing to change this for document pages 1-9
You will be relieved to know I did find a solution, unless I didn't find a solution I guess. I went back into Page > Apply Master Pages and re-applied the Normal L and R pages to the document page range 1-9 and clicked Okay. The offending header text was gone, greyed out header text frames, and both empty. Crossing my fingers, I exported the document using the holy grail PDF-x-1a. The pdf looks perfect, cross your fingers. Maybe I can stay out of your hair for a while.
I note that on about March 13th the discussion matter veered far from the original subject title (importing .docx etc.) into the world of Master Pages. Any lessons learned through Master Page fiascos might not show up to other pilgrims searching for similar enlightenment. Can you--or is there a set procedure for--moving the portion of this thread dealing with Master Pages to a place where it finds more daylight to those experiencing similar Master Page issues?