Did Abraham and Isaac actually possess the land God promised them?
No. Abraham and Isaac did not actually possess the promised land during their lifetimes. They continued dwelling among the Canaanite tribes, not owning the whole land. The promise remained future: God would give the land to them and their seed, but the actual possession was yet to come.
Les explains from Genesis 26 that when God appeared to Isaac, Abraham and Isaac were still dwelling among all the Canaanite tribes that were there when Abraham came in. They hadn't got the whole land, it wasn't theirs; they were still dwelling in the midst of the Canaanites. But the promise was that the day is coming when they won't have to put up with the Canaanites, and God is going to give them the land. In verse 3, God promised to give Isaac all these countries and multiply his seed, confirming the covenant was still future in its fulfillment.