Hey there! I had a very similar experience with a clutch a few years back now. In that case, the female laid a number of slugs over the span of a week or two, sort of at her own pace. The last egg took several days longer to pass than the others, but did pass naturally. I think without the 'momentum' of the eggs behind it, and the surface area of a normal egg for contractions to work against, a slug as the last egg in a clutch can be difficult to pass. Like yours, each successive egg seemed to be formed slightly better, but in the end, they all ended up infertile and oblong.
That being said, I would avoid oxytocin for sure. The efficacy of oxytocin in snakes is at best questionable, and in some cases (not all by any means) it has been suggested to cause issues like the rupturing of the oviduct and/or eggs, which can have fatal results. From what I understand, if it's effective at all, it would be in the very early stages of laying, say within the first several hours laying was induced. Aspirating the egg is a viable option, but should probably be done by a professional. I either case, I would give it several days before taking that step, and see if she passes it normally. I would also say try to gently but firmly milk the egg out, especially if it's near her vent. If for some reason it hasn't moved down the oviduct, it's probably something that can't be solved by manual milking. Try not to worry too much in these early stages though. Although a ruptured egg is a serious threat, I don't believe it's an immediate one with slug clutches. If weeks have passed and there is no progress, I would be be much more concerned. I think hormonal signals and contractions just become confused for slug clutches; slug clutches don't seem to follow all of the normal laying patterns. Anywho, that's just my advice, and should be taken with a grain of salt as I am by no means among the more experienced breeders on this forum! Good luck either way, and keep us updated!
Cheers,
-Matt