Since the Koran/Qu’ran is getting on for 1500 years old, presumably Arabic has undergone a few changes since it appeared.
The King James Bible, for example, contains at least a dozen words on every page that have all but disappeared from modern English, and were it not for its own presence we would probably have some difficulty in reading it.
My understanding, from my Arabic class, is that Arabic in its modern form is very close to classical Arabic. However, this refers only to Modern Standard Arabic (the school-taught/literary version). Spoken dialects vary greatly across the Arab world, to the point where an Arab from one country may not be able to understand the dialect of another country.
Hard to say. Classical Arabic may have undergone a mess of changes in the way it is pronounced. Vowels were pretty fast and loose and the old method of writing the language did not help much. Further, at least some words may have undergone a shift in meaning.
Currently the debate is over what the heck is meant by “usury” exactly.
Certainly Modern Spoken Arabic is closer to Classical Arabic than (say) Modern English is to early Modern English (think Shakespeare) as far as we can tell. But that does not mean that the two are mutually and totally comprehensible. Heck, if for no other reason than MSA has a mess of loanwords.
Some other sources list it as being as few as three. It’s another example of a linguistic continuum. If you travel across North Africa and the Middle East where Arabic is spoken, when you travel for a while you can notice that there is a different dialect from one village to one fifty miles away. You would be likely to say that it’s just a dialect difference though, since the two varieties are mutually intelligible but clearly different. On the other hand, if you have travelled five hundred miles, you notice that the language is so different that the variety spoken at one end of your journey is mutually unintelligible to speakers at the other end of your journey. So you might want to call them seperate languages.
So, in answer to the question in the OP, not only would the spoken language of the time that the Koran was written be unintelligible to modern Arabs, the speech of some modern Arabs is unintelligible to others. Of course, many modern Arabs have learned to read Modern Standard Arabic, which is a compromise between the modern varieties, or Classical Arabic.
I heard a lecturer who made an interesting point about spoken versus written language. As I recall, he said Arabic speakers who might not understand each other in spoken language can still communicate in written text because of their common script.
He also mentioned this occurs in China, where numerous rather different languages share a common script. In this case the sharing might be even easier, because the characters are conceptual, not phonetic representations.
In general, I gather that the use of written language tends to slow down change. The advent of large scale public record keeping apparently slowed down rate at which English is changing. So, for example, I’ve heard it said we have a much easier understanding of Shakespeare than he might have had reading Chaucer, even though Shakespeare and Chaucer were closer in time.
Presumably the common focus on the Koran has a similar effect on Arabic speakers.