I'm not sure that an add-in will meet all your needs but there's no harm in
experimenting (having backups saved<g).

As I said, just do a File, Save As with your workbook active. As the bottom
if the file type list in the Save As dialog is Add-in. Excel will volunteer
to save it in your "add-ins" folder but you can use any folder. After you
do the save the open workbook will appear exactly the same *but* there will
now be another file on your drive, but with the extension XLA. That's an
add-in twin of your workbook.

Now you can close your workbook and open the XLA. You will see nothing
because add-ins have no visible presence. But you can see it in the VBE and
change its code and resave it there if you want. As to linked workbooks,
they will not know about the add-in being created. You're going to have to
point them to the add-in by replacing the link to your XLS using Edit, Links
(and save them).

As I said, do this as an experiment. It may be a little weird working with
an invisible workbook but it will achieve your goal of being hidden.

--
Jim Rech
Excel MVP