So many interesting questions. I'll do my best.
Regarding Joshua 21:43: It must be that he was speaking of the boundaries made with Moses in Numbers 34, and not the boudaries made with Abraham. This is because right after this, in Judges 1, they were still trying to drive out the inhabitants of the land. Further, In Jer. 11 and Amos 9 both repeat prophecies about the land, which would indicate the land promises were not totally fulfilled in Joshua. And in Romans 11:25 ff, Paul still speaks of it as future.
Next, we must distinguish between the kingdom and the covenant, for they are not one and the same. The new covenant is indeed in effect today, as has been pointed out here. It is a new covenant in Jesus blood, which He initiated at the upper room, and 2 Cor. 3 and Hebrews says applies to believers today. However, the messianic kingdom has both a current spiritual application (My kingdom is not of this world (Jn 18:36); The kingdom is within you (Lk 17:20-21)) and it has a future literal fulfillment (when the Son of Man sits on the throne of His glory, and the apostles sit on 12 thrones judging the 12 tribes of Israel, Matt. 19:28). This is true because of the following:
--The disciples were still asking about it in Acts 1, and Jesus didn't correct them;
--Peter was still offering a literal kingdom in Acts 3:19-26 (When Jesus comes He will "restore everything")
--2 Tim 4:1 implies Jesus will appear and restore His kingdom;
--Many passages in the OT speak of a literal, national kingdom, with a government, borders, tribe of Judah, Mount Zion, etc. To make these mean different than what they literally say is to wrench the text.
So, most of the statements about whether the NC is currently established or still future are confusing the current NC with the future messianic kingdom.
Regarding the question "Why does Paul quote from the OT so much if it speaks nothing of the church," this is a non-sequitur. The conclusion does not follow. For example, just because Paul quotes from the OT does not mean that everything in the NT was predicted in the old. This argument would only be true if the OT quotes specifically speak of the church (which I think was Swaimj's original question). So this question does not answer Swainj's question. However, Paul does specifically speak of mysteries that were not known in the OT, and he specifically says in Eph.2 that the church is built on the foundation of the apostles, with Jesus as the cornerstone. So the church was not specifically known in the OT, but established with Christ and the apostles in the NT. All Abraham knew was that "through you all the nations of the earth will be blessed" but he didn't know how.
Regarding "if the New Covenant has been established dispensationalism crumbles," I don't understand how this could be, since most dispensationalists believe it is currently established. Again, don't confuse the NC with the messianic kingdom.
Regarding whether life in the church is like what the OT prophets predicted, the answer is yes, it is similar to it. But it is not the same as, since the OT prophets clearly spoke of a nation, forever, with the tribe of Judah, from a mountain called Zion, controlling the Nile to the Euphrates. So there are many similarities and many differences. The Mosaic covenant has been replaced by the new, but not the Abrahamic or Davidic.