Re: multiple queries to delete excess backup version w/ custom

[Richard Stallman]

    Another idea would be to turn off backups for the custom file, or to
    otherwise arrange not to ask the user questions when saving.

Those backups are important.  The fix is to make this operation save
the file just once.

Rewriting Custom-save might be a good solution.
Your first solution is ok too:

    One idea would be a global which tells custom-save-all not to call
    save-buffer.  Then, bind this when sending :custom-save to the widgets
    in Custom-save.