I think you might have missed the “clarification” that came out. She is heading the investigation into “Why” the immigrants are coming—not the activities at the border.
Harris’ people would not allow her to be put in such a position to fail.
I believe Vermont Lt is also correct in that Harris (and no doubt her handlers) came up with the counter strategy that she is going to work to determine the cause, rather than handle it directly.
All of which says that there are some Game of Thrones type stuff going on in the WH, that we are only aware of through analyzing their actions, as you both did.
We are living in those interesting times from the Chinese curse.