You're right.
Defending illegal criminals is totally unrelated to "Constitutionality." Hanged by your own petard.
Our Constitution is Silent about what defines a "natural born citizen," and "authoritative opinions" are all over the place.
Do you know the difference between The Constitution and ordinary laws?
Throwing the righteous phrase "It's unconstitutional!" displays the ignorance of the speaker rather than statement of fact.
The fundamental difference between ordinary laws and a constitution that emerged from the American Revolution was that ordinary laws:
a. described specific illegal acts, while a constitution granted positive rights.
b. addressed economic questions, while a constitution addressed the distribution of political power.
c. could be passed and repealed by legislatures, while a constitution was a fundamental law ratified by the people and superior to all legislation.
d. applied to the states; a constitution was a document of the federal government, and can be repealed only by formal amendment.
e. were approved by the people, while a constitution emerged from the decisions of judges.
LINK:
Ordinary Laws are not part of the Constitution!
So, no, it is NOT unconstitutional to challenge an illegal executive order or an ordinary federal or state law.