What an odd objection - you seem to be shooting blanks and
missing. On one hand, the galaxy not having any definite inside or outside is not a contested issue in science; stars and assorted matter thin out gradually, and not even monotonically, and never mind the dark matter or dark energy halo that further blurs the issue. On the other hand, the quite possible existence of "edges" (bow shocks, accretion belts, whatever) doesn't really affect this in any way: any "edge" of that sort may be considered located inside or outside the galaxy, depending on who is talking, because no unambiguous definition for inside/outside can be found.
"Where No Man" was written back in the bad old days when people of reasonable education and good writing skills still thought the galaxy is a cluster of light that borders on darkness. This is a blessing in disguise, as it makes the physical trappings of the events so unreal that they can be assigned any desired characteristics or interpretations afterwards without contradicting current scientific consensus. They are simply far enough outside reality to be immune to it. If Starfleet has found a concrete "edge", it's free to use that to define inside vs. outside, even if the definition will be rather arbitrary. But the reverse is not true: no inside vs. outside setup defines the "edge" we witness in the trio of episodes, and no particular location is established for it, save for "the edge is at the edge", or "it is where it is".