The standard career path is assistant conductor to conductor. I don't know about if a conductor hired laterally from a freight or commuter railroad can be hired on directly as a conductor, I suspect they can. Assistant Conductor trainees can and are hired off the street with no railroad experience, Amtrak will provide all training. They hold regular training classes. Amtrak OBS employees often transfer to T&E service as ACs, though they must attend and pass the training.
All T&E crew must be rules qualified and fully qualified on territories they work. They become territory qualified by making "student" trips on the route until they become qualified. I am not what the criteria is in determining "qualified", it may just be x number of student trips. There are a few detour routes where Amtrak maintains crew qualifications, one that comes to mind is the ex-SP East Valley line via Roseville between Sacramento (Haggin Jct) and Marysville (Binney Jct) instead of the ex-WP line. They maintain qualification by student trips on freights.
I am not sure whether, on a train that requires both a conductor and an AC, the AC can be a student under the conductor on and still be "on duty" or whether it requires a territory qualified AC with the "student" just along for the ride. I think it is the latter, and I know that on conductor/AC student trips they frequently ride in the cab.
As to taking over, whether an AC is required on a given train and the number of ACs required is a union agreement thing. From a rules perspective, if a conductor fell ill the train could could continue with just the AC, I am not sure about that from a union agreement perspective. I know that a train could not leave a terminal/crew change point without its full compliment of engineer, assistant engineer, conductor, and assistant conductor(s).