1) Trump can, but does not want, to declare a national emergency in order to build the wall. It is a very nice position to be in, really, knowing that he can get what he wants one way or the other. But he still wants it to go through congress. Why?

2) Because he wants a Republican congress and senate again after 2020. Right now, he's on track to win 2020. He's an incumbent in a good economy, and right now the markets both like a deadlocked government AND is now immune to the shocks that plagued the Obama shutdowns.