For getting this level of isolation, you probably don't need a separate process. A separate "AppDomain" will probably be enough.
For this, let either the calling code not link against your lib, or your library not link directly against
dll_A. Instead, load the WPF dependent libs at run-time into a separate AppDomain and use the .Net frameworks reflection mechanism to call the methods of your library, like shown in this older SO post, or this Microsoft page.
If you really want isolation on the process level, you need to put the code of your class library which is linked against
dll_A into a separate wrapper program (maybe a command line tool), implement some kind of interprocess communication (IPC) and provide a second class library which can communicate with the wrapper program and provides an API for the calling code. But beware, IPC always adds some overhead, this has always an impact on the performance and the required programming effort.