Will my upper trays crush the worms in lower trays or stop oxygen from getting to them?
No. Worms live in the ground in nature. They are trod upon all the time without being harmed. They also live under rocks and logs without getting squished. They can do this because the soil is porous enough to let them burrow and provides them with oxygen. It is important to mimic this porosity and aeration in the Worm Factory® composting system. The worms facilitate aeration of the bin as they burrow through and form little tunnels.
If the composter becomes too wet it hinders airflow. For information on how to improve oxygen circulation in your Worm Factory®, see Worm Bin Oxygen.