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Some plants can handle it, some cannot. The best thing to do will be to put the hot water into a large container so that it can be left out to cool prior to watering. This will prevent any damage from water being too hot.

Overhead watering could possibly cause a problem, especially if the water sits on them and does not evaporate. I doubt that one overhead watering caused this, but it's a better practice to water plants at ground level. Especially when they're small, it's the roots that need water. I don't know what kinds of plants these are or their environmental conditions, so it's impossible to be sure. Here's an article that may apply:
https://www.gardeningknowhow.com/plant-problems/environmental/leaf-scald-in-plants.htm

That used to be recommended but it isn't anymore. Apparently water doesn't flow well between different substrates.

One thing you can do, if the container isn't so large, is to create a pot within a pot. Use a lightweight, plastic pot as a liner and remove it when you water to let it drain.

Have you tried a water filter? Here's an article from the CDC that might be helpful: https://www.cdc.gov/healthywater/drinking/private/wells/disease/arsenic.html and here's an article from Gardening Know How:
https://www.gardeningknowhow.com/garden-how-to/watering/testing-water-for-plants.htm

I would suggest contacting your local university extension service for help with this problem. Here's a link for that:
https://www.gardeningknowhow.com/extension-search

Is it getting too much water? The roots may be saturated. Plants prefer deep watering once or twice a week, depending on the weather and the plant.

Sometimes, if it is really hot, they will droop in the afternoon, then perk up in the evening when the sun is going down. In that case, they don't need water.

Personally, I wouldn't. Many sources say that it is safe, but then they will go on to list many stipulations that could land you in legal trouble. It really depends on how important recycling water will be to you. If you are comfortable following the many strict laws, then it could be a good way to save water.

Make sure that you contact your local extension service before using graywater to make sure that you are not breaking any laws that could land you in trouble.

Here are two articles that will help:

https://www.gardeningknowhow.com/garden-how-to/watering/using-graywater-in-gardens.htm

https://www.gardeningknowhow.com/extension-search