A. DISM has a great /get-mountedwiminfo option that gives basic information about any mounted WIM, including the folder the WIM is mounted at and if it's writable. This is all the info you need to dismount it.

C:\>dism /get-mountedwiminfo

Deployment Image Servicing and Management tool
Version: 6.1.7600.16385

Mounted images:

Mount Dir : d:\Temp\mount1
Image File : d:\OS Images\Windows 7 Ult 64-bit\sources\install.wim
Image Index : 4
Mounted Read/Write : Yes
Status : Ok

The operation completed successfully.