Painting or sealing plywood is important if your layout is going to be in a damp area, such as a basement. Even with a dehumidifier, the humidity can fluctuate and cause warping as the wood absorbs and releases it.
From a scenery point of view, the plywood should be covered.
Most wood glues will work well with wood to homosote joints. I use Elmer's Carpenter's Wood Glue diluted about 25% with water to help it absorb into the homosote. Too much water will warp the homosote. For pine joints, like the layout framework, use the glue full strength.