Google Docs has not automatically added put all my headings into the document outline (this may be because the heading that is not included is right after the title). Is there a way to manually add a heading/point to the document outline?
No, it's not possible to modify Outline. It is created by "best guess" algorythm, and there is no option to change the way it selects content for Outline.
While you can't edit the outline itself, if you reapply a heading style, it will get added back, even if you delete a heading from the outline.
e.g. if you have an H3 which is missing, making it an H4 then back to H3 again will add it back to the outline. With the keyboard shortcuts this is quite quick.
Personally I'd prefer a "all headings only" mode (sidebar table of contents) and get rid of the flaky algorithm, but this is not possible at present.