And to you, sir, excellent questions!
A passport is an excellent form of ID. It is the only one you need (think of an I-9 form for employment; present a passport OR a DL AND SSN card). It is also a pain in the rear to get, since it is such a lengthy process dealing with the Fed Govt.
As far as leaving the country: she has been placed in the national missing persons database so she would not be able to cross the border into Canada (or Mexico) or fly anywhere outside the US without being flagged. Also, she could not take the children without MP's approval, e.g. a notarized letter of approval. Canada keeps coming up because she is from Buffalo--it's like our second home. But again, she would never get across the border.
Cash? If MP was changing the PIN numbers on accounts (according to ATRUEFRIEND) or perhaps anticipating a messy divorce, she was afraid of not having access to cash--especially without a job?
Hope this helps a bit . . .