That does sound nice, however, I had issues with combining the individual chapter files because there is an endnote list at the end of each chapter (which starts numbering at 1). It seems impossible to do an "odd-page section break" (to start a new chapter) after the last reference in the endnote list. After flattening, I had no issues combining.

When a cross reference is in the form of 7-9, it is no problem it only points to 7 and 9 and not to 8. I can't find how I can automatically change the format from 7,8,9 to 7-9. Do you know how to change that? Update: I found the shortcut key ctrl+shift+H, which works In case I would reverse it (change 7-9 back to 7,8,9), how can I do that?

In chapter 1 and 2, I changed in the note options "end of document" to "end of section" and also choose to restart at each section. Next, I opened chapter 1 and clicked the insert tab on the ribbon, chose the drop down triangle next to "object" and chose "Text from File". I selected chapter 2, which is then added after chapter 1. There are two problems, however. First, the headings from chapter 2 (e.g. 2.1, 2.2 etc) are not visible in the heading pane. Second, if I place my cursor at the end of the last reference of chapter 1 and press enter, I cannot insert a odd-page section break. How can I fix these two issues?

That's easily worked around by inserting a field coded as:
{IF{=MOD({PAGE},2)}= 0 {QUOTE 12}}
or
{IF{=MOD({PAGE},2)}= 0 "{QUOTE 12}¶
This page intentionally left blank¶
"}
at the end of the last endnote in a Section and using ordinary 'Next Page' Section breaks.
Selecting the range and pressing Ctrl-Spacebar should be sufficient.
You should insert a 'Next Page' Section break (or 'Odd Page' Section break if it suits your purposes) at the end of a given Chapter and, depending on your document structure, unlink the new Section's headers & footers from the previous one before inserting the next Chapter.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

I cannot do a normal section page either All section- and page-break options are grayed out.

If I select the ",7,8," in 6,7,8,9 and press ctrl+shift+h the result is 69. If I place my cursor between 7 and 9 and press the "-" key, I get an "_" instead. However, I can copy a "-" from somewhere else in the text and paste it between the 7 and the 9. If I then select 7-9 and press ctrl+spacespar it changes back to 7_9 and not to 6,7,8,9.

That's most likely because you're trying to insert the Section break within the endnote range. You need to insert it before the first endnote in the Section.Most likely what you're doing with this approach is deleting the ',7,8,'. Try inserting the '-' immediately before the 9 or immediately after the 6 before you hide the ',7,8,'.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

You are correct Thanks a lot! It works.

What I would like to do as well is insert a new section which has style 1 and which is called Summary. If I try to do that, word automatically puts "Chapter 8" before the word Summary. How can I do this without having it linked to chapter 7 (e.g. if you define a new level 9 style)?

The same applies for the Table of Contents: how can I have it appear in the heading pane above chapter 1?