Okay, two things:
One: I would be *very* surprised if this thruster truly violated conservation of momentum, as you say that's a fairly foundational thing. I would be much less surprised however if it only *appeared* to violate conservation of momentum by pushing against quantum vacuum, dark matter, the fabric of space-time, or something else entirely whose nature and interaction with "normal" matter is poorly understood or completely unexpected. And I would fully expect such a (working) device to be derided by the establishment as impossible until it was thoroughly proven to actually generate anomalous thrust, at which point many, many smart people would start trying to figure out WTF was really going on.
Yeah, I'm exceedingly dubious. If any of these kinds of mechanisms was so easily accessed then its very VERY unlikely we haven't seen it before. Not only that but this sort of interaction would almost surely be only one of a CLASS of interactions of various sorts. Also I don't think you can cheat by pushing against 'vacuum energy' (who's existence and nature are highly dubious) or other non-material things. I'm not really facile enough with GR, and I'm not sure ANYONE is to be frank, to say exactly what might or might not be allowed absolutely in theory, but it seems like a cheat to me, and nature isn't very fond of cheats. I think momentum will be conserved in its classical sense and I don't think any other sort of conservation suffices.
Two: I think you're not giving enough credit to past researchers. Until somebody noticed the anomalous exposure of some film that had been left in the same drawer as a mineral sample, nobody had any reason to expect the existence of radioactive decay - it was in fact impossible according to the then-current theories. That discovery shattered atomic theory (atom literally means indivisible) and opened whole new fields of science. Imagine the derision they would have faced if instead of only requiring some film and a mineral sample, duplicating the key phenomena required tens of thousands of dollars of equipment and testing apparatus. Nobody would have wasted their resources trying to duplicate such an obviously impossible phenomena, and nuclear physics might never have been born.
But you have to understand how far less developed high energy physics was at that time. VERY many experiments had never been done, so entire fields of possible phenomena were unknown. They couldn't 'balance the books' in even the simplest way, so they really didn't understand what they were looking at. Today we DO balance the books. We can analyze atomic and subatomic interactions and tell exactly what energy is going in and out, and in what forms. Its not an analogous situation.
>Its not circular reasoning to expect some phenomena to not exist.
Certainly not. It is however circular reasoning to extend that reasoning to conclude that NO unknown phenomena exist. And unknown phenomena will, by definition, appear to break the well-understood laws of physics, at least until the principles behind them are understood. And that does occasionally involve completely uprooting the existing laws, such as relativity did to Newtonian mechanics, which are now only kept around as an approximation that is accurate enough for most purposes - incidentally also a great example of simple, elegant physics being replaced by something far more complex and counter-intuitive. The universe is after all under absolutely no obligation to make sense to us, making sense of things is what *we* strive to do, the universe only offers an occasional hint of untapped mysteries if we poke it just right.
Well, when I claim that no unknown phenomena exist, then your argument is relevant, but since I haven't its really not...
I disagree that unknown phenomena will appear to break anything. Does the Higg's appear to break anything? No, it doesn't. Neither does Dark Matter, etc etc etc. They aren't entirely understood, but they don't break existing physics.
You're wrong about GR 'uprooting' Newtonian Mechanics. It did no such thing. It simply added a more refined theory. Not one single observation ever made under Newtonian Mechanics was invalidated under GR, though some new ones were able to be explained.