Simply deciding that the law was either covenantal or criminal does not end the discussion about what to do with the illegal immigrant...or the sanctuary city. There is yet another question that must be answered.
If you determine that the law that was broken is covenantal - that is, that it has to do with relationship more than with "right" or "wrong." Then you have to ask yourself how this broken covenant can be restored.
Essentially every law that God ever gives us in the Scriptures is a covenantal law. And overwhelmingly, the solution to the breaking of the law is some way by which the offender is able to come back into the community. This is true of the covenant between man and God, where God consistently pursues the faithful in order to bring them back into relationship with Him. This is true of the covenant between men, where God always seeks to figure out how to atone for the offense and bring the two back into the same community. It is true of the accidental murderer in the city of refuge. Only on very rare occasions is there no way for the offender to be restored to community, whether the covenant he broke was between men and God or between men and each other.
Thus, if you have determined that the law broken in the immigration issue is covenantal, then the next question to ask yourself is how this covenant relationship can be restored. (And note that God's answer is never, "Go back and do it the right way." God always meets persons where they're at and provides a way forward.)
But let's say that you determine that the law that was broken was criminal - that is, that it has to do with the ordering of society more than with communal relationship. Then you have to ask yourself whether the criminal law is a good one or not.
It offends most of our sensibilities to think about breaking a law at all. Most of us drive close to the speed limit, pay for everything we take from a store, register our vehicles properly, etc. But the testimony of Christian history is that sometimes, in order to do the right thing, we have to do the wrong thing.
For a long time, Christianity was illegal. Yes, illegal. Even before the widespread persecutions began, the New Testament records for us that the apostles were ordered not to preach the Gospel. Not for any reason. Not anywhere. Not to say a word about it. And their response? "We couldn't obey that law even if we tried." It was a law binding upon them, issued by the governors and courts, but they refused to obey it. (And that's how most of them ended up dead.)
Christians in Nazi Germany refused to obey the government's orders and assisted their Jewish brothers and sisters in escaping persecution. Churches offered safe harbor during the Civil Rights movement. Christians provided safe houses along the Underground Railroad. Over and over and over again, the history of the church is that it has stood on the front lines of social justice in the face of the law of the land.
So the question is - is immigration one of these issues? Again, I'm not going to tell you what to decide; that's up to you. But the question is: are the current laws regarding immigration good ones or are they ones that stand in the way of our living out grace and love according to God's laws (covenant laws)? We cannot just say the law is the law is the law; that's not what church history tells us to do as Christians. We have to determine the merit of the law and where it falls within God's plan for redemptive love.
These are not easy questions, but they are necessary ones. The issue, like all other issues, is not black and white; it's a thousand shades of grey. What are the laws? What kind of laws are they? Are they good laws? How do we reconcile them? What ways have been made for a man to come back? Where is community life in all of this? What is criminal?
What is Christian?