Welcome to GardenWeb! Agreed, it is sunburned, these plants burn very easily. Does the pot have a drain hole in the bottom? "I have moisture gnats." That's a sign the soil is staying too soggy, too often. This sogginess is harmful to roots, which need air as well as moisture, or they will rot. "For the last 2 waterings, I added dish soap (1T) and vinegar (1t)." Vinegar is often recommended as a "home remedy" for killing weeds, not something I would put anywhere near plants I like. "A few days ago, I sprayed with an insecticide supposedly safe for indoor plants." Those are usually made to eradicate the pests that kill plants, not necessarily gnats living/breeding in the soil of the plant. It might be a fine remedy, but unless it's recommended for that purpose on the label, I wouldn't use it for that, just for what it says it's supposed to do. "A day or two later, the flower completely drooped." Individual flowers don't last forever, so not necessarily a sign of anything except that it would be more natural for it to just turn brown, not droop. "Now the leaves are as well." In conjunction with having gnats, I fear the roots of this plant are rotting significantly, thus unable to deliver water to the foliage even though there is plenty (too much) in the pot. With all that's going on with this soil, I would take the plant out, rinse the old soil off of the roots, and repot with all new stuff, making sure it's in a pot with a hole in the bottom. Don't pack the soil tightly around the roots, and water when it seems like it's getting dry, not just because it's been a certain amount of time. As plants grow, and as the weather changes temp and humidity, the wind blows less or more, plants use water at different rates. If your soil has air pockets throughout, and doesn't hold onto excess moisture, and is not constantly remoistened, fungus gnats will be unable to breed in it. This is also what is preferred by the plants, so it's a win-win thing to do.

Mali, these plants are pretty much durable. One can grow them to look like they are barely hanging on. You can grow them to have many mishap-pend leaves. Or could could grow them flawless and in bloom. If you prefer the later, the best thing you could do for your plant is to use a very good porous mix in a container with holes simply put. If you are going to use the mix you are now which is very dense, I would not use much fertilizer without flushing your mix out at least once a month. I would never water in sips. I would not use tap water unless you are going to flush the mix once a month. And I would do frequent re pots at least every year to keep the mix from collapsing around the roots only to suffocate them. Looks like all this is happening to your plants as we speak. Remember, the more dense your mix is and the finer the particles in it, the greater chance of the roots suffering from a lack of good oxygen at every watering, salt deposits from tap and fertilizers. I will only use rain water on mine preventing leaf damage.

I've had mine go completely bone dry before finally beginning to wilt. This wasn't intentional, I only mention it to point out that the wilting isn't from underwatering. Peace lilies love water, but their roots also love fresh air, so drainage is a must. I would also advise giving it much brighter indirect sunlight. Just because some plants can tolerate low light levels, it doesn't mean that they prefer it that way. Never seek advice from a big box employee. For most of them, it's just a job; they're paid to be there. They don't care about providing accurate advice. You're better off seeking out someone shopping there that actually wants to be there. More often than not, that's where you'll find me, HaHa. I've certainly helped other shoppers whenever they needed it.

Most commonly, peace lily in steep decline are stressed by over or under-watering (usually the former), or root congestion, which is best resolved by dividing the plant (preferred) or repotting (as opposed to potting up, which only partially and very temporarily relieves only some of the stress caused by root congestion. There is no need to stand on ceremony when dividing. I use a small (12" handle) mattock to make the divisions, and use the divisions that come from the perimeter of the root mass, which will be more robust than those in the middle. Very often growers conclude, because PLs are often found in situ (where they naturally occur) in riparian (river/streamside) settings, they like or prefer wet feet/soggy soil. This is not true. While the plant can readily adapt to various forms of hydroculture, they can't tolerate the stress of back and forth from wet soil to almost dry soil. The adaptation requires a physiological change in root structure. When the plant is in a riparian setting, roots will be filled with aerenchymous tissues which provide a pathway by which oxygen from above the water line moves through roots to fuel metabolic root functions. Plants grown in pots develop a parenchymous root structure which facilitates extraction of oxygen from air-filled pockets in the soil, which means the same care should be taken to provide a well-aerated medium and avoid a heavy hand on the watering can. PLs require regular applications of fertilizer. Foliage-Pro 9-3-6 is a solid choice with rates and frequency varying according to grower's choice of grow medium and watering habits. Al