Many (most?)synth/controllers can send program change messages to all your remote sound sources. You need to dedicate a master unit that will send the proper commands to the respective modules, pc's, kbs.

A dedicated kb mixer seems a neccesity in this scenario as well. Do all the sub mixes at the kb location and send a mixed signal to the main PA.

I know that's noo too specific, but it's a starting block. See if that helps get you on your way !