She even tried to feed him a few times in his plastic carry box. Nothing worked
.It's not a out feeding in a small container that is actually counterproductive with a non feeder it's about having them live in something very small to promote security which lack thereof is the number one trigger I hatchling food refusal, the link posted above explain what and how an non feeder can be back on track and realistically if you buy a well started hatchling and it does not eat within the first 3 to 4 weeks there is likely an issue husbandry wise (keep in mind that yes there are exception and some animal less succeptible to stress but if a healthy hatchling will not eat for you that's is way of saying something is not working for me.

I understand it is not normal & stressful to the snake but how else would we get nutrients into him? Are you saying, if he won't eat to the point of starvation/illness, to euthanize him? Curious what you meant regarding the gastric juices. Interested. Plz explain
What would typically done would be to assist and forcefeed is the assist is a fail and that would be done at a rate of once a week which would offer enough food to sustain the animal until he eats and or recovers.

The stomach acid in snakes are a lot stronger than the average animal and stimulating them at such a frequent rate with an animal that is weak can only do more harm than good sadly.

As far as what you have described fluid build up, the dark spot in the intestine, weakness etc there is a high chance the animal could already experience organ failure with the regimen of antibiotics, bath and force feed every other day it won't help.