I'm still planting pepper seeds that my father - who died in 1990 - saved for his 1991 garden. They come up every year. Last year my oldest sister gave me a bag of seed packets. I don't know how she came by them, but they are ancient - one packet of cayenne peppers was labeled 1976, and there were coleus and petunias and maltese cross from the 1980s and early 1990s. I've planted all of these and so far have seen only amaranthus emerge. But it's not a tragedy since they were unexpected freebies anyway (and I have other coleus and petunia seeds that are newer). I always save leftover seeds to replant the following year. They're stored in the fridge between summers. Some don't last, but most do.

OK, so the thermometers aren't to problem. The puzzle of temp difference continues. In the link I referenced previously, they say: Radiation Cooling: On clear, cold nights, plants and other objects within a greenhouse will lose heat to the outside by radiation cooling. Because objects within the greenhouse are much warmer than those outside, they lose heat by emitting infrared radiation through the glazing and into the clear sky. Under such conditions, the foliage temperature may be 5 F cooler than the surrounding air. Radiation cooling occurs very little when skies are cloudy. If the same applies to the humid air trapped in the greenhouse, it could explain the difference. Since surfing the net trying to figure this out was more fun than doing my income tax return, off I went. This USA Today article explains why humid air (counter intuitively) is lighter than dry air. Ding! maybe lighter air radiates heat at a greater rate than dry air. Off I went on that tangent. I then stumbled across this Texas Gardener article on frosts/freezes that references the temperature differences between solid objects and the air around them... back to the point made in the original link. It further explains how wrapping / covering plants (as inside a greenhouse) will protect plants from frost. I'm more confused that when I started. I knew one day I'd regret reading comics during my Grade 2 Thermodynamics lessons. I'm going back to doing my income tax.

Peppers can be finicky in regards to fruit set. More so than many other plants but your conditions seem fine with regard to temp, pH and light. And you're correct that they are self pollinating but doing it by hand will usually give you a better yield. Given all that, either the plant is just not physiologically ready yet to handle setting fruit or it's missing something. I've had plants not want to set fruit and then one day there are fruit everywhere when conditions remained the same. But on the flip side of that I recently had a green chile plant that was doing the exact same thing as your pepper plant and I resolved it by hitting it with a couple very high doses of potassium. Pepper plants need a lot of potassium and since you're using a pepper mix I assume that it has higher K levels but who knows. I had blossom drop on that plant for weeks and within a couple days of hitting with higher K levels it started setting chilies. That's my 2 cents.

I garden a little north of Concord NH so I'm a zone colder. I grown all my peppers in the ground but my beds are built using the lasagna method. Each spring I top off the beds with a layer of composted horse manure. I usually add additional fertilizer to the planting hole - maybe an organic mix like Cheep Cheep. We have a high water table. Doing lasagna beds is like a raised bed without lumber. They are best built the season before planting because they will settle/compost a lot over a winter. I never use Miracle Gro products since I can usually get free or almost free contents for my lasagna beds. You could remove the soil from your beds and do a base of composting materials. Try reading up on hügelkultur beds, too. You can start with tree branches to build mounds. I think the advantage of beds of any kind is you don't walk on them which helps compact them and you don't need a tiller. Sometimes I have a pretty fresh lasagna bed so I make a hole in which I add composted manure and soil so I can plant something like a transplant or a large seed like squash. When I transplant my peppers, I water well with a liquid fish/kelp fertilizer mixed with water. I may repeat a couple of times while the plants are getting established. I grew some of my peppers in a high tunnel so due to the plastic roof, plants don't get much rain water except what maybe a little bit on the sides (sides are rolled up during the summer). This soil is drier than outside. We probably had similar amounts of rain to what you had. I don't think there's a big difference in how the plants responded to the rain or lack of it in the high tunnel. The high tunnel also heats up a lot with typical day time temps in the 90s. I know a lot of pepper lovers like pots but I'm also concerned about how much fertilizer leaches out of pots during rainy weather. I've been in gardens where cucumbers in pots were noticeably less healthy than those in a nearby raised bed. I grow a couple of tomato plants in pots. They always need more water and more fertilizer. Was it really wet feet that held your peppers back or the need for more nutrients? I don't think there's enough using any one type of soil or fertilizer. Some of the lasagna layers are kitchen scraps. We have lots of worms that we didn't have before. What's your worm population like? Good luck this next planting season.