> 1) c = Decimal64Context
> 
> 2) c = Context(Decimal64)

Rather that complicating the Context constructor, I'd prefer a separate factory function.  I was thinking of something like:

def IEEEContext(n):
    """Return the decimal<n> IEEE 754 context.  n should be a multiple
    of 32."""
    ...

Again, it's clear with this that you get a new context object (I agree that there are problems with (1) and the mutability of Contexts).