There can be something called hard belly in hatchlings... it's a problem where part of the yolk is hardened inside the abdomen. in cases of a small mass, it can be passed. Something this large would need a vet to remove. And even then, this is so large....
Is it soft or fleshy/fluid like? If it's fluid there's a chance it may disappear on it's own, but I have never seen a hatchling look so swole.
The downside with everything is that this could be due to some sort of deformity or organ issue because of the incubator issue.
If this was my baby, I'd really be on the fence about a wait and see or bringing it to a vet and trying a bit of a drain to see what the fluid actually IS. Clear fluid would probably be another wait and see, blood would be an euthanasia. Assuming it's fluid. If it's a hard mass, I would probably be considering culling/euthanasia just because surgery on a neonate like this is already risky and with such a large mass probably extremely taxing on the body. These are my thoughts on the matter at least. Perhaps someone else has experience with something like this as well with more insight
It's a tough choice. If the little one is at least active and alert you'll have a couple days to figure out where to go from here. If it seems lethargic or weak, euthanasia would most likely be best.
I'm sorry that this first experience has been so hard for you. I'm hoping that if you do try again that things are successful. I would like to know what happens if you do try and see what's up with this little one though