Google Grants Eligibility & Program Details
As long as you are not trying to "game" the system (which you are clearly not) you really do not have anything to be worried about. Google wants you to have this grant and Google wants you to use this grant to its fullest.
As for ad disapprovals, those actually come from the AdWords team, not the Grants team.
The ad you have posted certainly looks fine and would fall within mission-based guidelines, especially for a museum such as yours. The only issue I see is that the headline is over 25 characters, but content-wise it is fine.