Most of the time the heart is mentioned in the Quran, it can be replaced with "mind." Arabic simply uses the organ of the heart to represent the mind, notwithstanding whether there is a real biological relationship between the two.
As for your first question: If it is a metaphor, how can the location be mentioned?
This is not a metaphor. A metaphor is defined as a comparison between two objects without the use of words of comparison (e.g. "like").
This is a metonym, which is when one thing (usually physical) is taken to be a literary representation of another thing (usually more abstract). There is often some sort of link between the two which led to people making that metonym. An example is how "White House" is a metonym for the US government.
The organ of the heart in Arabic is a metonym for the mind. As such, specifying the location of the heart adds emphasis, but it doesn't change the fact that the physical heart is itself a literary substitute for the mind.
The mention of "in the chests" here is to emphasize the fact that the blindness being described is something hidden inside them, not something in their outer body.
As for your second question: The relationship between the heart and mind is based on a false understanding of biology, so why would the Quran use it?
Using a word or idiom in a language is not an endorsement of the theory that led to that idiom being created.
It was simply idiomatic in Arabic to refer to the mind with the heart, and that is not affected by what theory this relationship was originally based on.
It is similar to how heart in English is a metonym for love or emotions or aspirations. E.g. "Follow your heart." Using phrases like these would not be considered a mistake or endorsement of the faulty ideas which originally caused this relationship between the two concepts.
Even if someone said "Follow your heart" while pointing to a person's chest or someone said to be poetic "You haven't left any scar on my body, but you have left a scar on the heart in my chest," that would not be an endorsement of any physical link between the heart and emotions.
Rather, all of that is simply using the language as it is, even if the reason the language came to be like that was based on a faulty understanding of biology.
The Quran did not come to teach people biology or rearrange their understanding of human anatomy. To expect that from the Quran would be missing the point.