It's possible to control the MPA server by sending commands over a TCP/IP net ? - what are the messages to send : can we do all the functions available in the DLL?

yes, the optional available MPA-3 external control software contains a program mpa3net.xe that allows to send commands via TCP/IP to the MPA3 server. All commands described in the command language section of the manual or the online help ("How to use the command language..") can be sent and a status information can be obtained (see description of the commands ending with a quotation mark on page 5-25 of the manual).
The functions in the DLL cannot be used.