DHS isn't going to shut down no matter what happens. The USCIS is funded through user fees, not appropriations. The rest of DHS pretty much qualifies for an exemption from shutdown under the "emergency services" exceptions. So, CBP, TSA, ICE, and other DHS agencies will continue to operate. The only parts that will stop working are the nonessential administrative components. Bottom line: No matter what happens it will be business as usual.