For question (1), the best you can do I think is to write the text you
need at the end of a paragraph, or something, and make it transparent,
and very small. Perhaps writing it in a text frame which you then attach
as an anchored object would work too, I haven't tried. Another thing I
haven't tried is to add your text as "conditional text", and hide the
condition. But I suspect that the running header wouldn't pick up on
hidden conditional text. Again, I haven't checked.
For question (2), if you separate each chapter into a separate story,
then the master page for the chapter openings won't move by itself. But
it does mean that if you make edits, you may end up with overset text at
the end of the chapter, which you will then have to deal with manually.
If that is happening a lot, you may want to consider switching on Smart
Text Reflow in Preferences (but use with care).
Alternatively, if you do not want to split your document in separate
stories, and you want your master pages to be linked automatically to
paragraph styles in your document (each Master A = Chapter Heading
paragraph style, etc.) you can try out the Mastermatic script, which
allows you to link paragraph styles and object styles to master pages.
You can also set it to update automatically:
1) Right-click a page (in the pages palette) and select "Numbering & Section Options..." There, you can check the "Start Section" box and input text into the "Section Marker" field. On your master page (containing the running header), you insert the "Section Marker" special character, just as you would the "Current Page Number."
2) I believe the best solution is to manually add 2 pages to the end of the chapter you're adding text to, then re-flow the text onto the new pages.
Sorry for the late response but my PC was for repair (which luckily was successful...).
(1) Bot solutions don't really work for me.
@Ariel: I would need to add such a text along with each paragraph on all pages the header text does apply for since otherwise the running header does appear only on that page where the heading text is provided (once). Apart from that it would be a possible solution because the heading moves along with the paragraph.
@Robert: That solution is smart. However, it applies for the whole page but should apply only from certain paragraph within a page onwards. The header text should move along with the text and not be anchored to the page.
Is there anything else I could do on that?
(2) is solved. Thanks Ariel.
I contacted Adobe support and I was informed that Indesign cannot handle point (1) - Microsoft Word can
Nonsense. I do this all the time by adding a non-printing frame with the correct text at the point I want the running header to change.