Here's a picture of a recent flower that fell off. If you look at the stem, you can see the beginnings of a pepper. The soil is wet, because I had watered the plant.

Here's a nighttime photo the jalapeno.

Only 4 (too much water) seems probable, but again I'm not sure. How dry does the container soil need to be in between feedings? As for low humidity, plenty of people grow pepper plants indoors year-round in low humidity so I'm skeptical of this as a possible cause. And as for 11, I've lost about 2 dozen flowers that failed to fruit so now I am stumped.

The plant looks very thin and weak overall, so I'll assume it isn't receiving enough light energy for strong production. Eventually, you should get a meager set of pods.

Josh

There's a difference in the amount of light needed for survival and the amount needed to flower and set fruit. I know many people who grow pepper plants as houseplants and with the exception of the guy who has a sunroom, they don't tend to get much fruit set. I am growing jalapeño now and the plant in the picture does not look like it is thriving. It looks like it needs more light. Given the other picture of the soil surface and the fact that you mentioned adding more compost I'd say you are likely over watering as well. Houseplants especially need to dry out between watering. And that's what you have right now... a houseplant. So if you want to keep it going then treat it like that: well-drained, non-compacting potting mix, water soluble container plant food every few weeks, and watering when the plant needs it. I would even look at supplementing with grow lights because it could definitely use it.

I'll let the plant dry out more before watering and see if that helps. Presently, I water when the first 2" of soil is dry. As for the soil, it's in a well-draining, non-compacting potting soil with additional perlite. As for the sun, it's in a south-facing window. I'll move it to another south-facing window and see if that helps some. I think I'm puzzled by the fact that the plant is flowering regularly, but not fruiting. Thanks for the input.

As previously mentioned, blossom drop is caused by a number of environmental stresses. Its basically the plant aborting the fruit when conditions are suboptimal. It happens to me every year at the beginning because nights are a bit colder than optimal. I What makes me suspect the plant is not getting enough light is that it looks leggy (longer than normal distance between where each leaf stem or branch is attached to the main stem), with a single thin stem towards the lower half, and sparse foliage. Normally by the time my jalapeño starts flowering and setting fruit there would be way more branches with more leaves on each branch, and the main stem would be thicker relative to height. Windows block a surprising amount of the sunlight, so even though you have a south facing exposure you are no where close to the 6-8hrs/day of full outdoor sunlight recommended for growing peppers. The legginess especially is a good indicator of insufficient light. What concerned me about watering is that the soil surface in the picture looked too wet, which peppers in general do not like, and which is the main killer of houseplants anyway. Indoor plants do not suffer the Temps extremes or wind and tend to use a lot less water. You might consider experimenting to see how long it takes for the plant to show signs of wilting. Then you can water. But first pick up the whole plant, pot and all, so you can get a feel for the weight dry. Also the use of compost in containers, especially houseplants, is not common because it tends to hold onto moisture and compact over time, and as there's no active soil webs in the container to break down what nutrients it might provide, it doesn't really have a reason to be there. Not all potting mixes are created equal; there are a lot of bad ones out there. Some houseplants will tolerate slow drainage and compaction. Peppers really don't.

I feed the plant epsom salted water every so often

That can cause too-low pH from the sulfate and also magnesium toxicity. EPSOM SALTS IS NOT MAGIC!