That might seem to be the most likely culprit. However, unlike descriptions I read online, the plants don’t wilt: They behave like they have some sort of plant gangrene that moves up from the lower stem, and the stem once affected appears to mostly dry out. The top half (3/4, 1/4, whatever is left) looks reasonably healthy, it’s just “static” (other than fruits dying almost as soon as they form) until the dying / drying gradually consumes the healthy portion from below. Somehow the “healthy” portion is still receiving just enough water and nutrients from below to hang in there. I was even thinking of taking a cutting and trying a “restart”, although I’ve never successfully rooted a cutting from any plant of this type...
Gardening is one of the most rewarding yet frustrating things you can do! I don’t know what else could be damaging your Opo. I had borers one year, but finally quit growing squash all together because I couldn’t deal with the squash bugs any more.
As an aside, one of the most ‘fun’ things I’ve done regarding damaging pests was to hunt tomato hornworms with a black light - spooky with the garden in purple, but a great way to spot them.
Per link in 678:
Organic controls include wrapping the lower stem with nylon stockings or aluminum foil to prevent egg laying, which generally occurs within a couple of inches from the point where the stem emerges from the soil. Row covers can be used up until bloom. An old gardener trick for vining squash and pumpkin cultivars is to cover the vine with earth at various points along its length, inducing rooting at several points, thereby continuing to feed the developing fruit despite the loss of the original stem. However, this only works with the species Cucurbita moschata. After the vine has taken root at multiple points, the infected portion of the plant can be cut off, along with another inch where the larva is eating into healthy tissue, without significant damage to the plant. Some gardeners choose to place a yellow bowl filled with water and a drop of liquid dish soap to attract and drown the adult pest, a method which also provides feedback on the current level of infestation. This method, along with daily checking the plants for eggs, is popular with organic gardeners.[3]
Once larvae are present within the stems, control must be quick. Often, the first noticed sign of the infestation is wilting of the vine, and it is too late for control efforts at that point. Experienced gardeners watch the stems for signs of frass protruding from small holes, which is an indicator of the presence of the larva inside. Some gardeners carefully cut the stem along its axis and remove the caterpillar before it ruins the vine. Others use a stiff wire, a needle, or a toothpick to kill the borer without seriously injuring the vine.[3]
Have you tried rooting hormone? It’s a powder, dip the cut end in it and plant.