Hello,
I would recommend just removing the H3 over the map overlay. There is no way to specify a higher z-index to the Google Element to overlay the H3 text that I can see. The H3 text also prevents the users from clicking the "X" button if it is overlaying the H3 container.
.module_round_box-centered_googlemap h3.s5_mod_h3 {display: none;}
Note that you can also add that H3 information that was at the top to the overlay block in the bottom right corner if you wish.
Regards,