Interestingly, your own extended quote from American Jurisprudence contradicts your thesis, and clearly lays out why "legal" and "constitutional" are not separate terms, as a purported law that contravenes the Constitution, "though having the form and name of law, is in reality no law" and "is as inoperative as if it had never been passed". Therefore, while Congress could pass something that pretended to be a law which violated the Constitution, by doing so it would make "in reality no law", and whatever was purported to be made legal by that law would not be legal.

To be legal, under US law, an act must necessarily be Constitutional as well; an unconstitutional act is, ipso facto, illegal.