Since each proof has a possible weakness, does the Murex case collapse?

Not every kasha is a shayla.

A local question on one proof does not erase the whole case. The Murex argument is cumulative. It includes geography, ancient dye industry, shell remains, dye chemistry, the kala ilan comparison, classical sources, and the Greek/Latin/English translation tradition.

That is how many real-world identifications work. You rarely get one source that says, “This exact species is the chilazon of Techeiles.” Instead, you weigh whether all the lines of evidence point in the same direction.

Chosam Shel Zahav states this kind of cumulative approach directly. It argues that if we find a known dye-producing chilazon from that period and region, producing a color comparable to kala ilan, and known from classical sources, then it is hard to posit a second unknown chilazon that did the same job while leaving no comparable footprint.

Bottom line:
A kasha deserves an answer. But it does not automatically become a shayla on the whole identification. The anti-Murex side often treats every non-absolute proof as if it becomes worthless. That is not weighing evidence. That is dismantling pieces without looking at the structure.