Taking on the MyDMX software:
Thr 3D Visualizer needs to be as accurate as possible. While I am all for configurnig real lights on-site, the whole purpose of the 3D Visualizer is to save this hassle. If the 3D Visualizer is reasonably accurate, for me, that works. Most of my shows are concerts and since people move around and generally don't share stage directions with me, I can at least get something basic and hit certain marks. It's not ideal, but since most of the acts I deal with either stay put or move around too much, I've been able to get by. If I'm working a choreographed show, then typically I got some follow spot operators that the promoter hired(and popping my FOH circuit!)
But, back to the issue: it's sounding like a long microphone cable and/or no DMX terminator is the cause of these issues. It's consistent with the type of description being provided.