Yes. Currently, there seems to be no pre-Islamic reference to Mecca. That might change. But there is, according to Anthony, sources from the 7th century attesting Mecca.
In the Quran, yes. I incline to say like Reynolds that the Quran is more ancient that usually thought.
I am not sure why we need to assume that non-Arab sources are dependent on the testimony of the Hagarenes at all. Sure, they could well be, but the significance of the sources to begin with was their early and independent testimony. But you could be right.
They are. They did not know nothing about what the few informations ("prophet" "Muhammad"...) recounted by the "Hagarenes".
Concerning Dye, you misunderstood what he was trying to say. Dye argued that Q. 19 originated in Palestine (i.e. outside of Mecca, and this also applies to other so-called Meccan suras). He never argued that the entire Quranic corpus originated in Syro-Palestine. In fact, he does not deny that parts of the Quranic corpus do indeed fit perfectly in Western Arabia (Ḥijāz). He even says that he sees no reason to abandon the notion that Mecca existed before the advent of Islam. According to him, Mecca did exist, but was not a major trading center, nor was it a major city, but might have been a city that of religious significance. To be fair, he does say due to the obscurity of things pertaining to Mecca, there is indeed room for debate. He also claims, and rightly so, that the existence and size of Medina (Yathrib), as a city playing a role in early Islam, is more certain.
If you have a twitter account, ask him if he agrees with your statement.
"According to him, Mecca did exist, but was not a major trading center, nor was it a major city,
The traditional account says the exact contrary about "Mecca". And Dye knows it very well. As it is "impossible" for an academic to say what I say here, it is the better he can do to lower considerably its role. He knows as well that saying this, is saying that the traditional account is inexact : how what Dye describes can feed, clothes, equip 50 000 strong to wage war in Palestine and Iraq ? It is not credible, and he knows it.
But for Ibn Ishaq , it is necessary and logical for him to describe "Mecca" as he does (great and rich city, etc) since he believes that the conquests are the fruit of the Quranic proclamation in "Mecca" and
departure of the conquest to Palestine and Iraq which involves food, clothes, equipment and troops and therefore implies obligatorily a great an rich city of commerce.