But in the absence of any other information, larger numbers of similar specimens do at least point toward a regularity of some kind, and tend to eliminate the accidental. Whether that regularity was due to deliberate manufacture or some natural process would still be a problem.
One problem I have is that design may be only a singular event. Things I have designed over 10 years ago are still being made, and I've seen some recent examples even though I've moved hundreds of miles from the point of original design, and the manufacturing has since moved as well. This makes tying the objects to the design difficult, and all you have then is the artifact.
No problem. If the artifact has elements that do not tend to occur in nature you can infer design. The more such elements the better the inference.
Whereas a nice projectile point is clearly "designed" even if you find only one, and it is away from any cultural deposit.
But do you really have evidence of design, or of manufacture that copies a design?
Doesn't matter. A nice projectile point can easily be differentiated from the natural. Regular bifacial flaking is a nice feature of man-made points. Bifacial flaking is rare in nature, and doesn't look the same when it occurs.
But we, as archaeologists, don't care about the "design or copy of a design" issue. It'll poke a hole in the side of a deer or elk just fine either way. And I suspect the deer or elk doesn't care much either.
But why are you bring up all of these irrelevancies? Design vs. copy of a design? Why do you think that matters?
Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.