Basically no gadol outside Radzyn agrees with the cuttlefish being the chilazon.
R Herzog proved that the blue color comes from chemicals , not the ink of the cuttlefish. The Radzyner Rebbe never knew the color came from the chemicals, in fact, in ‘Ein HaTechelet’ 1:22 , p.288 and ‘Ptil Techelet’, p.168 he explicitly says that the chemicals do not produce the color, so we see he was fooled by the chemist.
The Beis HaLevi, the Lubavitcher Rebbe Rashab, and the Aruch HaShulchan among many others already ruled against the cuttlefish.
To your two statements about the chilazon being a fish or snail and the supposed lumdish answers the need more pilpul, you need to start from the beginning.
- What does the word chilazon mean?
It is not a proper name, but the name of an animal in Aramaic. Any Aramaic dictionary like Jastrow would tell you that it means “snail”. So perush hamilot is the basic of any good exegesis.
Now let’s go to what the rishonim further say:
• Rambam in the Perush Al HaMishnah in Judeo Arabic (Keilim 12:1)
חלזון- והוא כעין צדף עשוי מברזל נועלים בו את הדלתות, ואותו הצדף הוא צדף של בעל חי הימי הנקרא חלזון
Chilazon - It is like a shell made of iron, like the shell of the sea creature that is called Chalzun in Arabic.
• Rabbeinu Gershom (Berachot 38b), Rashi (Avodah Zarah 28b)
״חלזון קורים בלע״ז לימץ״
“The chilazon is called in foreign language, limace (Old French for snail)”
לימ״ץ בלע״ז מין חלזון
“Limace (Old French for snail) in a foreign language is a kind of chilazon”
• Chochmat Shlomo ( Shabbat 77b)
שבלול לימצא נ״ב הוא פי׳ של חומט
רש״י פי׳ שבלול חומט שקורים בל״א שנע״ק
Shablul is a limace (Old French for snail) and it is the meaning of chomet. Rashi explained Shablul as Chomet which is called in another language schnecke (snail in Yiddish).
• Radak (Hullin 132a)
שבלול הוא שרץ הנקרא בלעז לימס״א,ויש אומרים קראקו״ל…שבלול שהוא מתכסה בתוך קליפתו שהיא לו כמו לבוש
Shablul is a sheretz which is called limace (Old French for snail) in a foreign language, and there is who call it “caragol” (snail in Spanish)… The Shablul hides itself inside his shell which is like a garment.
Finally, see the Vilna Gaon on Eliyahu Rabba , Keilim 10:1 brought in the ArtScroll Talmud which says that “anything in the sea is a kind of fish whatever its form may be”.
- Regarding the word yam being interpreted as bottom of the sea
Check in the Tanach Habbakuk 2:14 and Isaiah 11:9 for how the water covers the “sea”, obviously the sea floor.
Furthermore, see Deut 33:19 how the chilazon is a hidden treasure of the sand and Menachos 44a where Rashi says the chilazon comes up from the earth and the Yavetz clarifies that it refers from the sea floor.
I hope these answers help you in your study of this sugiah and in identifying the chilazon to fullfil the mitzvah of tzitzit in a complete manner.