I'm not aware of any passage in the Bible which spells out that God meant "legitimate heir"
Because there aren't any in this case. I believe as you do it has everything to do with Issac being his only son by his first and official wife. But this isn't in the bible either. The subject of favouritism - some suggest nepotism - of one child over another is all to do with the rightful - first born heir of the official wife as was the rules back in Abraham's own land of birth Mesopotamia . He was simply adhering to the rules of ascendancy. It is as simple as that.
For christians to even acknowledge this brings them great problems simply because the god of Abrams god - as he was named - is a different god to that of the hebrews and the Christians. When Abraham was ordered to up sticks and travel to Canaan. he arrived there neither being Hebrew or Jew.
Abraham was aware of the conflict and perhaps questioned it, but said nothing.
Not mentioned in the scripture so again we have to submise and assume. Not acceptable to christians
Abraham believed God would still fulfill the promise, just without Isaac.
Not mentioned in the scripture so again we have to submise and assume. Not acceptable to christians
Abraham somehow knew Isaac's death was never a possibility and that God would intervene.
Not mentioned in the scripture so again we have to submise and assume. Not acceptable to christians
Abraham believed God had the power to even bring Isaac back to life once the test was over.
Not mentioned in the scripture so again we have to submise and assume. Not acceptable to christians
A combination of two or more of these.
Not mentioned in the scripture so again we have to submise and assume. Not acceptable to christians
As I have already explained; all the above would be quite good and reasonable explanations for what is, on the surface a silly unexplainable story that doesn't show this so, so loving god in any-kind of favourable light.
The Mopecs and ethang5's of the world simply refuse to accept anything that is not in the scripture -- UNTIL IT SUITS THEN TO DO SO.
As for prosser, he often makes his stance clear of what he believes concerning the "unprovable thousands of years old scriptures" because we weren't there and puts them all down to BS . Yet when I started a thread on Joseph he was the first to tell me who Joseph's god was although the name of Joseph's god is never revealed in the scriptures.