Learn English – Metaphors similar to “Trojan Horse”

They're related, but generally a metaphor is used to draw a comparison between two distinct objects, whereas a symbol is used a stand-in for a much more complex, and generally more abstract, idea. In literature, a metaphor would typically be used in a specific instance to compare two objects, but a symbol would be used throughout the work as a major part of the theme.

No, There is no imperative to create a new construct here.

When a metaphor ''loses its metaphorical properties'' due to the demise of colloquial meaning of its components, where the meaning remains we call this an idiom. Where the meaning is lost entirely, we call it a dead metaphor. Both of these descriptions are accurate, depending on the time, place and persons from and to which the phrase is spoken or written.

Also I believe the following phrasing manipulates the outcome of the discussion:

'high point [e.g., of a career]'

The above definition includes his very own implied assumption of "high" being strong or successful! It would be far more accurate to simply use the following phrasing in our definition of the zenith of a career:

'strongest or most successful point [e.g., of a career]'

I believe this is simply a discussion of the difference between metaphor and idiom.

In the case of the phrase "Falling in Love" we're really looking at an idiom. Whether or not a person understands this phrase is a combination of 1) whether they've heard that particular idiom, and 2) whether idiom exists in their native paradigm at all (e.g. certain languages have no idioms, and the concept of idiom can be a challenging new one).

However, this is completely separate from the fact that a limber mind can interpret meaning in places where it has not been literally defined. We call this metaphor.