Re: shouldn't customize-save-variable raise an error if var is not an option?

[Richard Stallman]

    I don't know the answer, because I don't know if
    `customize-save-variable' is intended to "work" by program on
    variables that have not yet been declared using defcustom.
    I can't find any doc that speaks to this.

I don't know.  However, given that `customize-variable' will operate
(if you force it to) on a variable defined thus

    (defvar foo45 nil "*Foo you")

and does allow saving a value for that variable, I guess it is
reasonable that `customize-save-variable' also allows that variable.

I do not see any bugs in what you described.