The information and documents obtained to identify your client may be kept in your client file. There is no need to maintain a separate file.

Yes, as long as a paper copy can be readily produced.

You have to keep the information for the longer of the duration of your professional relationship with the client, as long as is necessary to provide service to the client,...

Yes. The rules require that you obtain and retain a copy of every document you used to verify a client’s identity.