As the title states, on a given event (for me this happens to be upon opening a new google.maps.InfoWindow I want to be able to close any other currently open info
You need just one line:
$(".gm-style-iw").next().click();
There's a div with this class, 'gm-style-iw', inside the infowindow. After, there is the div of the close button. So, this code will click every infowindow close button existing in the map