Patur- Thank you for responding. I will answer your first point now and will bezras hashem get to your other points a little later.
1- About the similarity between kla ilan. All I was saying that according to Rabbi Miller, whom you quoted to hold that the Murex is Kla ilan, than you cannot say that they only made a gezeirah on the cheap stuff being that the murex is also kla ilan. All I was pointing out that you were soiser yourself.
But regarding your sevara that they would not make a gezeira on expensive stuff, it is very not mistaber. The gemara in menachos tells us of a test to tell the distinction between kla ilan and the real techeiles. The gemara tells of us two ways of doing the test. One way if the color of the dye were to fade than it was kla ilan and if it stayed the same color it was techeiles. Than Rav Ada had an alternative way of testing it that if the color improves than it is techeiles and if it stayes the same color than it is kla ilan. On 43a the gemara recounts how Mar Mashchi brought techeiles to Rav Achai and Rav Achai tested it and it passed the first test but failed the second test. The Rabanan thought to pasul it. Rav Achai asked How could it be that it is not techeiles or kla ilan rather you must say that the tests are meant to be together. Ayin shum vdoik.
What we see from that gemara that they knew that there was no other dye that is the same color as techeiles except kla ilan. And today we know (based on the ???? ????? ????? of that time) that this snail was rather common in those times and was used to dye blue. So being that the chachamim were not choshesh for it, obviously anything that is identical to kla ilan and sticks strongly, and was around in the times of the gemara is clearly techeiles.
And please do not bring up your old argument that this is the same chemicals as kla ilan, because we have already discussed that it is not the same strong so obviously a test can differentiate.