Several quick comments as I don’t really want to get bogged down in this:
1. As for the dating, the late is historical and traditional, back to the earliest notes of the church. If we are wrong, then all of history is wrong. If we are right, then you are wrong. I have not read Gentry’s book but I understand that there are what many consider to be huge gaps in it. Perhaps one day I will get a chance to read it. I think Thomas dealt with that issue pretty well in his commentary. Ken says,
And, of course, futurists must cling with all their might to the late date as without it their position collapses into dust. Both postmills and amills can maintain their eschatological positions regardless of the date of the book of Revelation. This is not entirely true however. With the late date, you must deny the prophetic nature of revelation for a posttrib rapture. That is thin ice to be on. I think the positions are very similar in that regard.
As for the kingdom, we must look at several things. 1) When we see the kingdom in Scripture, considering all the references, what do we see and what should we be looking for? 2) How did the first centuries of the church treat the idea of the kingdom. As to (1), there can be no doubt that the OT kingdom was not spiritual but literal and earthly. Alva McClain
The Greatness of the Kingdom lays this out very well in Israel’s history. One problem seen here often is that people fail to distinguish between the universal kingdom of God and the mediatorial kingdom of God. The vast majority of kingdom references are to the mediatorial kingdom, instituted at Sinai, mediated through the Davidic lines, and promise to be restored. There are simply too many OT passages that must be made into ground chuck in order to get anything else. I am uncomfortable doing that to the words of Scripture. As for (2), the early church, almost without exception, was premillennarian. It was not until the 4th or 5th century that other views crept in. When they did, it was because the traditional understanding did not seem to be taking place. In other words, people getting their theology from current events, i.e., “We expected an earthly kingdom; it hasn’t come yet; therefore we must have misunderstood.” And it is always the dispensationalists accused of reading theology in light of current events.

In any events, the post and amill views were later views that arose from confusion about the current state of affairs. Again, we must drive our stake in the text of Scripture, not the disillusionment of those who did not see the promises they thought they would. After all, if the OT saints didn’t do, why should we???
Do we have the authority to insert 2,000 years? I would answer that it is not a matter of authority since we haven’t inserted anything. However, there is no problem with 2000 years since the telescoping nature of prophecy is well known. Consider the OT prophecies of the coming of Christ. Putting them all together without benefit of the NT, we see only one coming. We do not see two. Therefore, we are entirely consistent with Scriptural precedent to admit that there are some things we simply do not know. We must simply believe what God has said, the promises he has made, and live accordingly.
I don’t think Pentecost figures as much as you think it does Ken. Remember even after Pentecost in Acts 3:19-21, Peter preached for the Jews to repent so that the Kingdom could be restored to Israel. He specifically says that Christ must be received to heaven until the period of the restoration of all things. In the context, it is hard to avoid seeing that as the Kingdom of the OT. Yet it is clear that Christ would go to heaven, until the Jews repent such as prophesied in Zech 12:10ff. as well as other OT passages such as the New covenant passages. When that happens, then will be the time of the “restoration of all things.”
This is longer than I intended, please forgive me.