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It really depends on the plant, but in most plants, bent leaves are a sign of both either being overwatered or underwatered. What kind of plant do you have? If you let us know, we can let you know what the proper watering cycle is for it.

This article should help: https://www.gardeningknowhow.com/plant-problems/environmental/what-causes-brown-edges-on-leaves-of-plant.htm

I don't know of a lid that you can buy, but you can make one. Take a look at using an old tupperware lid of the correct width to cut and place over the soil.

Well, I would definitely not chop it down the middle. Would it be possible for you to send a picture of this plant? This would help us to correctly identify it so we can give you proper pruning/trimming information for this particular plant.

This article will help you: https://www.gardeningknowhow.com/houseplants/hpgen/getting-rid-of-mushrooms-growing-in-houseplant-soil.htm

I could better answer your question if I knew what kinds of plants you have and how long you've had them, and what kind of light they're in. When there's something wrong with your plants, look first at the soil moisture, then the light, then look for bugs or disease. In general, watering regularly is far less important than knowing about the moisture in the soil, down where the roots (the heart of the plant) live. Learn to check the soil moisture in the bottom of the pot before you water; use an electronic moisture meter. Or a thin wooden dowel or kebob skewer, insert into the soil, pull it up, and run it between your fingers. Depending on the type of plant and the light it's in, the meter should read somewhere between dry and damp, and the probe should feel from dry to dlightly damp. Then you can water again, and water enough that you get a run off from the drainage holes.
Generally speaking, when leaves fall off plants, it's either because the light is too low, the soil is either too wet or too dry, or there is disease or bugs. This article tells you a bit about that: https://www.gardeningknowhow.com/houseplants/hpgen/desperate-houseplants-dealing-with-common-houseplant-diseases.htm