Rain checking/weather checking leading to black shoulder. Not a well understood problem. I've been seeing a lot of people report it this year, though. Bad if you're a market grower, but if you're growing for home use, it's just an aesthetic issue. It's not zippering. Zippering is covered on that link as well and has a very distinctive look and a different cause than checking.

Some of the leaf yellowing also looks like normal leaf aging as well, especially in the third pic. But there does appear to be a disease in the first two pics. Based on your photos, it looks like bacterial speck, which I get in my garden. But it could also be septoria. The response should be the same though: spray a copper fungicide regularly and prune off foliage at the first signs of disease. Don't let it sit on the plant the way it is now. You can alternate between Serenade and copper weekly, Re: bacterial speck and septoria--they're definitely far from the worst diseases you can get. Septoria doesn't affect fruit at all, for one, and speck doesn't always affect fruit. Both are relatively slow-moving diseases. You can usually keep a plant with speck or septoria alive and bearing until the end of the season. If you do end up getting spots on your fruit from speck, it's cosmetic and only on the surface, and the fruit is completely edible. As long as you don't have to sell it at market, it's not that big of a deal. The standard recommendation, however, is to never use diseased fruit in canning, as compromised fruit can have an altered pH, and proper acidity is important for canning safety. Re: nutrition--your regime sounds fine, so I would suspect the yellowing is from a combination of the disease and normal leaf aging.

Abnormal locule development is a result of some form of genetic manipulation that affects the ovary early on in development. That gene mutation can be either intentional or accidental like the use of auxin or gibberellin supplements, intentional manual cross-pollination with another variety, internal ethelyene gas development due to environmental factors (since it appears to be way over-ripe that is a real possibility), etc. But this one looks almost parthenocarpic which would indicate the presence of some sort of A or G supplements. But without the name of the variety we don't know if it was supposed to be bilocular that is just malformed or multilocular that only developed 2 instead (that would be my guess). That knowledge would narrow down the possible causes toward helping prevent it in future plants but either way it won't help this (these) fruit. Dave

Make sure it is above 70 degrees where it is growing. Also, the plant looks very large for that small container. I recommend re-potting it and try to get it in the ground ASAP. Something similar happened to my tomato seedlings, and after I re-potted them, they did better. After putting them out in the garden, they really started looking healthy then. Be sure that before putting them out in the garden, you harden them off by placing them in the shade and slowly move them out into the sun over a week.