Your solution is innovative and shows a lot of understanding of Word.

I would suggest using a bookmark name other than "list." That term may cause you problems in documentation.

Your code is fields.
Fields are, by their very nature as a part of a document body, fragile. Bookmarks are even more fragile. Bookmarks are very easily deleted or destroyed even when the contents are displayed.

Can you put the field including the TOC in a Section of the document which is protected from editing? I believe this would mean that the only updating would be automatic upon printing. I have not tried this, just making a suggestion.

Otherwise, can your train your people to select an entire area when updating the TOC so it will include your preliminary fields? Perhaps some innocuous symbols that mark the beginning and end of the area to be updated?