1] ... because the Rambam [Tzitzis 2/2] says that tchelis comes from a fish ["דג" - also the name of the famous Boston College QB Doug Flutie who threw that Hail Mary on November 23rd 1984 to Gerald Phelan to beat Miami 47-45. זכותו יגן עלינו] and our tcheilis comes from a snail??
Answer: The Gr"a in his Peirush Hamishnayos [Keilim 11/1] says "כל שבים מין דג הוא" - Anything that comes from the water is called a "fish". The Chavos Yair calls our snail a fish "דג שנקרא דג הפורפור".
[פורפור is the older name for what we today call Murex Trunculus].
2] The gemara in Shabbos [75a] says that if one is צד [traps] חלזון [from which we extract tcheiles as the Tosefta in Menachos says תכלת אין כשרה אלא מן החלזון] on Shabbos he is חייב. If it is a slow moving snail like our Murex Trunculus, then apparently there should be no איסור of צד [because it is ניצוד ועומד - pre-trapped, due to it's slowness]?
Answer: Rabbeinu Yeruchem [נתיב י"ב חלק י פ:] says that there is an איסור צד on snails and the like. So paskened numerous contemporary poskim [see שלחן שלמה שט"ז ד, תורת המלאכות ח"ה קנ"ו ועוד ועוד].
3] The famous claim of the Beis Halevi [the first "The Rov"- even before anybody heard of "Torah and Madda"] that that fact that we had it for all of the generations and yet didn't wear it created a tradition that it is not the correct tcheiles?
Answer: This argument doesn't apply to Murex because there was a decree that it may only be used for royal garments so we didn't have the option of wearing it and it was thereby forgotten.
4] The Shulchan Aruch [9/5] says that the tzitzis should be the same color as the garment and if our tcheiles isn't the real thing then we lose out on fulfilling the words of the Shulchan Aruch??
Answer: The Rema argues that we don't follow this opinion. The Yeshuos Malko [סימן א ב] says that there is no איסור if one wears ספק תכלת.
5] If it is not the correct tcheiles then one is עובר on בל תוסיף??
Answer: The gemara [ מנחות מ] says about one who wears קלא אילן [bogus tcheiles] "לא יהא אלא לבן" - it is like wearing white and the Rishonim understand that this means that there is no איסור involved.
