Can I send / retrieve MMS messages via WiFi?
It could be done technically. AFAIK all communication between MMS enabled components is done via standard TCP/IP.
But in a regular provider setup the MMS store and forward server(s) (MMSC) are only reachable via the providers network. That's why you can not send and receive MMS via WiFi.

It's also the reason why you can not make a call and send/receive an MMS simultaneously when you have only 2G (GPRS/EDGE) connectivity.

Normally android will drop the GSM data connection if there is WiFi connectivity and all data will be routed through the WiFi interface, but there is one exception: The MMS traffic will still go through the mobile data connection.