Literally seems to have simply been overused as a hyperbole.
When a hyperbole, a style figure that can be very powerful, is used too often, it loses its impact. This is a common phenomenon.
I remember when it was common to say you were giving a 100% (of your effort) to a cause. That was already a hyperbole, but once everybody seemed to be giving their 100%, people started giving their 110%, and by now some people are going for it, 200%!
As the hyperbole gets overused, it loses its effect and its meaning.
In the case of literally, it has been overused so much that people consciously interpret it as “ok, what follows is meant to be interpreted figuratively”.
As for amateur, I don't see any similar issue.
In describing someone as an art amateur, you stay close to the original meaning of lover or enthusiast. In this case, the opposite of amateur is one who is simply not interested.
In describing something as amateur work, you contrast it with professional work. In this case, the opposite is professional.
For those two meanings of amateur to be seen as antonyms, you should somehow be able to relate disinterested with professional, but I fail to see that link.
So, amateur has different meanings, but I fail to see how they are converse.