Well if you think you can get away with that bit of twisted legal logic then by all means go for it. I do agree with your description. But in reality that excuse will sink faster than a lead duck legally speaking.
Also, in Oct. of '05 both houses of congress enacted a joint resolution (H. CON. RES. 274) clarifying congress' stance on Posse Comitatus.
Excerpt from the resolution:
"by its express terms, the Posse Comitatus Act is not a complete barrier to the use of the Armed Forces for a range of domestic purposes, including law enforcement functions, when the use of the Armed Forces is authorized by Act of Congress or the President determines that the use of the Armed Forces is required to fulfill the President's obligations under the Constitution to respond promptly in time of war, insurrection, or other serious emergency."
The current caravan of illegals headed through Mexico to the U.S. combined with recent spike in illegal border crossings would fall under the category of "serious emergency" to all except crazed leftists who aim to eliminate our borders and sovereignty. ...like those on the 9th circuit. Thankfully SCOTUS is now composed primarily of constitutionalists.