Did Abraham and Isaac ever actually possess the whole promised land?
No, Abraham and Isaac did not possess the whole promised land. They continued dwelling among the Canaanite tribes, and the land was not yet theirs. Scripture records that they were still in the midst of the Canaanites, but God promised that the day would come when He would give them the land.
Les explains from the text that Abraham and Isaac were still dwelling among all the Canaanite tribes that were there when Abraham came in. They had not obtained the whole land; it was not theirs, for they were still dwelling in the midst of the Canaanites. But the promise was that the day is coming when they will not have to put up with the Canaanites, and God is going to give them the land. Les then reads verse 3, where God says, 'I will give thee all these countries, and I will perform the oath which I swear unto Abraham, and I will make thy seed to multiply as the stars of heaven, and will give unto thy seed all these countries, and in thee shall all the nations of the earth be blessed.' Les points out that everything has to come through this one little nation, and that every word of the Book came out of the nation of Israel. He notes that all the writers of Scripture were Jews, and Paul makes this plain in Romans chapter 3, where the advantage of the Jew is that unto them were committed the oracles or the word of God. This makes the Book unique. The Abrahamic Covenant comes in here, and in Abraham's seed shall all the nations of the earth be blessed, because Abraham obeyed God's voice, kept His charge, commandments, statutes, and laws. Les then moves to chapter 46 to deal with Jacob, the next in the line of Abraham, Isaac, and Jacob.