It's Very Much a part of my theory. Just not one that's welcome much in these parts.

It goes like this: They are not merely strongys but the superworm we have heard so much about.

They are:
a) resistant to everything we currently have
b) not just strongy's, but gifted with some additional abilities, as you have said: the ability to remain within the host for entire life cycle, which is HUGELY adaptive.
c) The ability to remain within host, as you described, with no secondary host or environment, makes me think you're not necessarily dealing with strongys....I would need to see pics.
d) A unique population. They must be handled on a one-on-one basis, as a few of the strongy papers recommend....Europe has stopped making OTC anthelmintics in order to slow the rapid development of resistance. The US has also restricted several effective compounds. This is common knowledge.

My theory says parasite population will increase with increasing treatment, since adults can be as small as a thousand cells. When you "kill them"...or more correctly, paralye them, they are either digested, or they fall to pieces, thereby reproducing a thousandfold (parthenogenesis...think tapeworm)....if you do not shit them out fast enough, while they are paralyzed, you have a bunch of clones, alive and well...and much more resistant to whatever you just attacked them with.

That suggests that your fecal material is hanging around in your colon for WAY TOO long. (3 days to get to maturity for worms, who are somehow surviving with zero oxygen).

And then, how do the lil' b*stards make it from the large intestine all the way back up to the small intestine again? I mean, that's called swimming against the current....and if you've paralyzed the little rotters, they really can't swim so good...that's my thought.

I think you're giving these guys too much credit