Genesis 50:22-23 Joseph's descendants

"Joseph stayed in Egypt, along with all his father's family.  He lived a hundred and ten years and saw the third generation of Ephraim's children.  Also the children of Makir son of Manasseh were placed at birth on Joseph's knees."  (Genesis 50:22-23 NIV)

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Reading these verses brings several questions to mind.  I do not  know if I have the answers, but we can still explore them a bit:

• I wonder why Joseph and his brothers had not left Egypt in the past.  I mean, I understand that they couldn't have remained in Canaan after burying their father Jacob, for they could not just leave their children and flocks behind in Egypt.  Yet I wonder, why hadn't they left Egypt before, some time soon after the famine ended?  The only ideas I had were that perhaps Jacob was not physically up to another long journey, or perhaps Joseph would not have been permitted to leave Pharaoh's service.  At this point it is still before the time when a later Pharaoh would enslave the Israelites, so the Israelites were still looked upon favorably.  Perhaps Joseph felt constrained to stay and help rebuild the nation after the seven-year famine.  Of course behind it all is the fact that God had told Abraham that his descendants would be enslaved and mistreated for 400 years in a foreign country.  (Genesis 15:13-16)  God hadn't specified which nation would enslave them, but it is possible that God had revealed to Joseph that the Israelites should remain in Egypt.
• I also wonder about why Jacob and Joseph both adopted their sons' two sons.  I guess I need to understand more about the social customs of that culture.  I think these adoptions were more of a formal honor than anything, which allowed these sons to inherit portions of their grandfather's estate.  I notice that it is Manasseh's sons who are honored in this way.  Jacob had indicated to Joseph that his son Ephraim, though not the firstborn, would be more powerful than his brother Manasseh. (Genesis 48:17-20).  Perhaps Jacob seeks to honor Manasseh in this way to show that the selection of Ephraim had been because of God's choice, not any type of favoritism on Jacob's part.  Any other ideas?

I should note that the fact that Joseph died at the age of 110 would have great significance to the Egyptians.  We will discuss that more in the next portion of Scripture, which covers Joseph's death.