Hello Marc,
Glad the Wurrd app and Mibew are working for you. Given your setup you should not have any issues. You can embed an HTTPS button on an HTTP website just fine. When a visitor clicks on the button, the frame that opens up is an HTTPS frame so the communication is secure.
Note though that when creating the button you must enable both "Include host name into the code" and "Use secure links (https)" to ensure that the button always links to the HTTPS version of the Mibew server.