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You've already hit the proverbial nail on the head. Peppers are hottest when grown in hot, dry environments. When you can control the weather, let me know... The only thing I can suggest is growing in containers where you can have more control over soil moisture, and grow in a greenhouse where you can let temperatures get into triple digits. A pepper grown more slowly and 'poor' will have more hot capsaicin oil, therefore, it will be hotter.

It sounds like a virus, my peppers did the same thing. Without lab testing of a sample, it's difficult to determine what specific virus has done the damage. I just destroy the plant because it can't be successfully treated.

There are some do it yourself recipes for pest control. This article lists a few:
https://www.gardeningknowhow.com/special/organic/natural-home-pesticides-organic-garden-pest-control.htm

You can also look at neem oil. It is very effective and organic.

It may be scale, which is another kind of pest. Here is information on them: https://www.gardeningknowhow.com/plant-problems/pests/scale-bug-how-to-control-plant-scale.htm It may also have been caused by the chemicals in the soap sitting on the plant too long. When making insecticidal soaps, try to use a soap that is as basic as possible. Many people like to use Murphy's Soap for this reason.

It's probably caused by insufficient pollination, or by weather fluctuations -- too hot, too wet, then too dry, then hot again etc. Shouldn't affect the taste, though.

Here is an article from Cornell University on heirloom seeds which might answer some of your questions:
http://smallfarms.cornell.edu/2014/07/08/cultivating-the-heirlooms-of-tomorrow/