It is frustrating.
You don't have to copy words for words to plagiarize ideas. You can tell the story of the same person, but with different details.
There are no doubts in my mind that Muhammad used biblical characters in biblical stories without having to read a single word.
No one said that the Qur'an was word-for-word copy of the Bible. It is stupid to think so.
You can take a story, and change it.
Clearly the stories of the bible and other Jewish legends and folklore were well known, that people didn't need to read the stories to learn it.
No where in the bible state that Solomon could understand the speech of birds or ants, or them understanding human speeches, but it does appear in the Qur'an. But Muhammad clearly Solomon's abilities, not from the bible, but from ancient Jewish oral traditions of folklore that were later recorded in the rabbinic writing, known as the Aggadah.
The Aggadah were derived from mostly the Midrash that were composed during the 1st and 2nd centuries CE, and some were derived from the Talmud. The Midrash and Talmud both come from Jewish oral traditions that predated Jesus.
What is clear that Muhammad's version about Solomon's abilities were definitely not original story. Muhammad was not stupid nor deaf, so he must have learned from Jews when he was younger, because there were Jews living in Arabia centuries before Muhammad's birth. Muhammad didn't grow up in a cave.
One thing is certain, that you don't need to read, to learn a story. That's Muhammad's version about Solomon is not exactly the same, doesn't mean much, because you don't have to learn word-for-word in order to tell a story.
Take the story of Adam for example. A number of gnostic (unknown) authors wrote about the creation and Adam that depart from the original story. Muhammad have done the same things, except that he learned story relating to Adam, more from listening to another person telling it, not reading it.
The story where Satan refused to bow down to Adam when God created man, doesn't appear in the Genesis, but it does in the oral tradition/Aggadah, and in the Qur'an. The story that won the naming animals appeared in oral Jewish folklore (Aggadah) and in the Qur'an, clearly showed that Muhammad heard of it being told, not reading the writing.
Even when stories are written, they aren't always exactly the same, because some authors/scribes will modify it at some points.
The story of Gilgamesh have been copied and rewritten so many times for over 2000 years. Some details are not exactly the same. The standard version found in the library of Nineveh (7th century BCE) bear a lot of similarities to the Sumerian poems, but it is also quite different in details and styles. They are certainly not written, word-for-word copy.
Stories can change over time, whether they be oral traditions or written versions.
