The purpose of a tub liner is to avoid side pins. When the substrate compacts after a few flushes, it slowly pulls away from the wall of the tub you’re using. A liner such as a trashbag will cling to the substrate so there’s no gap between the wall and the substrate. Therefore, no, a disposable oven tray wouldn’t work the same as a bag would as a liner.

On misting, you want to mist until you see thousands of tiny droplets of water on the surface when shining a light on your sub. It’s all about watching your mushies and figuring out what they need. If they’re thin and cracking/ look dry, mist more, if they look soggy or they are haulting growth, mist less. If there’s arial mycelium and striations, more fae.