único - placement and article use to alter meaning

Question

Similar questions have been asked before, but I wanted to take this one step further.

As I understand it, the placement of "único" alters its meaning. Preceding the noun it means "only" while following the noun it means "unique." However, if I wanted to say "only child" as a complete description I would revert to "hijo único." Now, this is where I have a question. I'm assuming using "hijo único" acts as a complete description rather than just a noun + adj. and therefore an indefinite article would not be required in the Spanish version of "I am an only child", whereas if I was to say "I am a unique child" this would be a noun + adj. construction and therefore the indefinite article would be required in Spanish. So to summarise:

"Él es mi único hijo"="He is my only son"

"Él es mi hijo único"="He is my unique son"

"Soy hijo único"="I am an only child"

"Soy un hijo único"="I am a unique child"

Are all these correct?

Answer 1

1) "Él es mi único hijo"= CORRECT. You say that "he" is your only son/daughter

2) "Él es mi hijo único"= INCORRECT (at least really strange)

3) "Soy hijo único"= CORRECT. You say that you have no brothers or sisters

4) "Soy un hijo único"= CORRECT (but the meaning of the sentence is different than three). Here you are comparing you (as son/daughter) with all the sons/daughter in the "world".

Gracias txustaboy. So is 2 *technically* correct, but just a strange thing to say? In English it is a bit strange too, but it does reflect these two sentences combined: "He is my son. He is unique." - jellonz, Nov 11, 2015