I think you need to shift your perspective a bit. Your current comments and observations seem to come from a church/Israel centered view. The issue at hand involves a much greater dealing of God with His creation.
God shared with Abraham about His plan to deal with the nations in Canaan but states that the time is not yet full. This is about 430 years before He sends the children of Israel to take over the Land and possess it.
In a similar fashion, God talks about judging Egypt and setting the Jews free and in so doing bringing the gods of the Egyptians under judgement.
In the prophets, God warns about the impending judgement on not only Israel and Judah but Edom, the Amorites, Assyria, Babylon, Egypt and many more. If you look carefully, these warnings often start decades or even centuries before the judgement is enacted.
This method of God's will is explained in Scripture of warning about impending judgement through His prophets and demonstrated over and over.
As for coming out from among them, the Zac Poonen series in question appears to be from the 80s or 90s which I believe came at a time when he was moving from a much more legalistic view to one of Spirit and grace filled. My understanding of legalism is that it is very focused on the purity and sanctity of an individual as maintained by that individual. Because this group must maintain their own sanctification, it is nearly impossible to have any contact with the outside world. A Spirit filled and lead individual, though subject to the same temptations, is safe to go and be anywhere the Lord directs just Christ was on the Earth.
So the idea of "coming out from among them" and being in the Land, there is a very separation from all that is the wisdom and character of the World. The Land can be compared to the Church but first it must be compared to the individual's life. Just as the people had to be set apart before entering the Land and the related rest, so must we be utterly set apart to God before we can enter the rest. All those born in the wilderness had not been circumcised and God would not bring the people in until they were. If the people are the means through which the Land was sanctified, one could compare them to one's spirit or soul, the source of will and purpose that brings action. If your will and purpose is not fully set to realize the Word and all the blessings and goodness it promises, you'll stop before you get there. However God told the people it wouldn't happen instantly. It would take until they were big and strong enough to fill and possess the Land. This is warning and promise of God as we seek sanctification and maturity. He goes before us to bring us there but we must be patient and advance in faith without compromise careful to not make any agreements or overlook any related sin. There are similar warnings to the churches in the New Testament regarding sin.