Short Answer? You can't.
Tableau is generally the "end game" when it comes to data analysis, so while the tool can do exporting, we tend to focus our time on other (arguably) higher-value asks that customers have -- not a good thing in your case.
You'll find that Tableau's export capabilities are (generally) "all about the data", not the titles / images / whatever that wrap the data. We don't attempt to export a full-fidelity version of a dashboard to Excel as there's no reason to.
Not the answer that you're looking for, but hopefully it'll save you some time. Sorry.