What I'm trying to accomplish is simple: I want custom headings 1 and 2 (which are also on outline levels 1 and 2) to appear in the TOC and as PDF bookmarks, whereas heading 3 (which is on outline level 2, not 3) should only appear as a PDF bookmark.

I found out how to exclude heading 3 from the TOC, but a new issue has arisen: I can't change a heading's outline level. It just doesn't work and is always reverted by Word.