The statement "I did not drink alcohol till the blood test" is a necessary statement to validate the blood test. Even if I never drink alcohol, it is necessary to make if the test requires such period of abstinence. But the statement "I did not murder my neightbor till the blood test" is nonsensical because there is no relation between my blood and the murder. Perhaps my use of "I" in the examlpe is misleading because of course I know about myself both before and after the test. Remember, it is Matthew, who does not know the intimate life of Joseph and Mary speaking to us. It is exactly analogous to a nurse speaking to a doctor: "the patient did not drink till the test". The nurse knows the condition before the test because of the chemical analysis. The nurse does not know, and is not interested, in the condition after, so he is not speaking about the after. Matthew knows the condition before, because he has a statement form Joseph (for example) made at the time of Jesus's bith. He does not know and is not interested in the condition after that.
Son of a carpenter, fine. Son of Mary, fine. Brother of James, Joseph, Simon and Judas, NO WAY!
But this is consistent with the fact that a man has one father and one mother, but many relatives of the same generation. "Brother" can be used expansively; "mother" cannot. Besides, "son of carpenter" is indeed used imprecisely here, just like "brothers" is used imprecisely.
Even using your reasoning then, Matthew could not have believed that Mary was a perpetual virgin, because you say he did not know the condition after. Wouldn't he have then assumed a normal relationship, if someone had asked him? I still think that "until" means "until", and that Matthew would have necessarily known about Jesus' bio-half brothers because of the time he spent with Jesus. Of course they would have discussed their families, right?.
Besides, "son of carpenter" is indeed used imprecisely here, just like "brothers" is used imprecisely.
I don't see it as being the same thing at all. Matthew is reporting what the people said. The people could not yet "conceive" (Ha-Ha) that Joseph was not the bio-dad of Jesus. The identity of siblings, though, was easily knowable.