OK I found the processes, they were started by mavros.service, mavros-serial.service and mavros-serial2.service scripts. They were NOT disabled by /home/erle/apm.sh script. I disabled them and will try to start the Pi0 in 'normal' WiFi mode, not an AP.
BTW, it's nowhere mentioned directly in Docs, that Ardu*.elf binary running on the drone is actually a client program, the Ground Control Station is the server, and it's GCS who actually has to listen to UDP port 6000. So if you run it on a desktop computer, be aware of Firewall and allow inbound connections!..