First, let me say I am impressed by your determination. Not giving up is an important part of success. Second, I will qualify my comments by saying I have never had a separate monitor board. In the “old days” of analog the task you are trying to accomplished used a special snake that basically functioned as a splitter. allowing you to plug a mic into it to go to the main board and there was also another socket that feed the monitor board.
in the digital world Allen & Heath uses what is called a Dante card (extra add on at $1500 each) in each device. It does not appear that the QU-16 will accept a Dante card (someone correct me if I am wrong) ….
The other option is make your life much more simple and just send monitor mix outputs from the GLD-80. you can run 12 outputs from the AR-24 (may need some for mains and subs) but you could easily go 8 or 10 outputs. That would be my recommendation.