I now see what you want. My problem with understanding your point is with your model.

You are not looking for "X=Z, Y=Z, so X=Y" An extended metaphor as you call it relates X and Y to Z as I have done. Dew is not necessarily flower related, but it is never face related. You need the flower to put dew on the face.

You are perhaps looking for X+Y+Z= Big Idea.

I think what you get is X+Y+Z = Confusion

The example you have mentioned

Love is gentle, and love is kind --
just like a jewel when first it's new.
But love grows old, and waxes cold,
and fades away, like morning dew.

Is a horrible example of MIXED metaphors. First love is a person, then a jewel, then what? a combination of the moon and a candle? (shouldn't it be wanes colder?) Then something that fades away, Which dew does not. Unless there is a metaphor making dew equal to light or colour. Each thing is only tangently related to the other and even then each description of love is at best questionable.

The author of this piece is forgiven because it is so emotional and pretty when sung. Emotion is the key word here. The author has layered a number of pretty mental pictures together to create emotions of longing, regret and confusion. The metaphors are pretty but uncomfortable when strung togethjer. One is actually relieved to get back to the boat. The only ideas in the song are that "love is difficult and I want to get away" These are very simple ideas.

I'm not sure what you mean by big ideas but, In answer to your question, Mixed metaphors (I'm using the term mixed because that is how my teachers defined what you are describing) are a very poor way to communicate complex ideas. They just add confusion and uncertainty.

For a good song which communicates a lot of information and complexity see "Hurricane" by Bob Dylan. It criticize racism, civil rights and the justice system without actually saying there are problems. The song is pretty much free of metaphor. The information and ideas are carried in the narrative. the emotion carried in the choice of words and how Dylan sings them.

For a song communicating ONE BIG idea see "The Times they are a Changing" Also by Dylan with just the one big metaphor of the "floodwaters of change"

If you want to communicate a Big Idea. Keep the song simple and focused.

If you want flowery language (please pardon the metaphor) then you may throw in lots of metaphors. But then clarity is next to impossible.