It is not your fault that QoS is not working properly. There is a known bug that QoS doesn’t work properly when used in conjunction with netbalancer.
Regarding your second question about the topology, it is not exactly an error but your users are able to change their IP and go out of any DSL they want, thus avoiding any QoS rules you might want to enforce in the future. My suggestion is to install another ethernet card on the box and connect the 2 DSL modems directly on ZS, using the switches only to connect clients.