Hi mima, Boron (B) is slow moving inside the plant, so symptoms of deficiency show up first in the older leaves. B is much involved in cell membranes & the transport of Potassium which maintains the water equilibrium of tissue (ie: your plump varicose midrib). B uptake is less efficient in situations of: drought (many pepper growers keep 'em dry), low Nitrogen (N provides the vigorous growth that can move micro-nutrients), high Calcium & also when pH is high (may be pH rose over winter). B is very mobile in the soil & prone to leaching. It merits thorough mixing of fertilizer blend for ideal bio-availability. Deficiency symptoms for certain crops vary & most think of B problems on flower/fruit/tips of growth. As regards leaves, Boron deficiency can cause them to: distort, cup, curl at margin, pucker, deform, shrivel, crinkle, thicken, brittle, dry & appear wilted (when on young leaves begin to see yellowing &/or central leaf vein chlorosis the B deficiency is already long standing & crop yield will be less)

Yellow leaves and purple veins usually indicate a phosphorus deficiency. This could be corrected organically with worm castings or a little bit of bone meal. Watering plants in containers washes out some of the minerals, so plants in pots are more prone to this problem than plants in the ground. Here is a link that might be useful: What's Growing On?

looks like a fungus and could be early blight. Had a cool wet spring and early summer so Im battling this with the tomatos and to a light degree on the peppers. The peppers seem far more resistant to it. I thought I might lose some tomatos but now with 2 weeks of dry weather they are creating more new leaves at a much faster rate than leaves are getting infected. Im suspecting Im dealing with Septoria Leaf spot which is more controllable than early blight. What ever you do, avoid touching other leaves particularly of any other plants after removing the sick leaves. I wear nitrile gloves, cleanly cut off the whole leaf and stem, NOT letting them fall on the ground if possible since you might just be adding more spores back onto the soil, toss them in a bucket and throw them in the trash. Remove the gloves and wash your hands before going back to the garden

I hope you shaded it after transplanting. For the winter inside it lost it's hardness. 18 inches tall and wide doesn't sound too big. My peppers by the end of the season are usually at least 2' high, may be more, and at least 20 inches wide (more, if I let them be.) you can trim it a little, I guess. But not the branch that is producing. Support that branch, tie it somewhere, it can easy snap under wind. Other than that, nothing at least 2 weeks after transplanting, then you can start fertilizing...