So how did Abraham know the condition of a child’s heart at 8 days old? It is ludicrous to think that every child born to Abraham was in no way a member of the Abrahmaic covenant, prior to faith.

Abraham did not have to know the condition of his child's heart in order to circumcise him. God told him to do it, so he did it. Circumcision told nothing necessarily regarding the person's faith. Only those with faith are in the Covenant of Grace, so not all those circumcised were in the Covenant of Grace. They were not true members of the covenant but only apparent members or members of the Covenant Community. The true sons who had faith were the heirs of the covenant promises. In contrast to his brother, Jacob inherited the promises and blessings of Abraham. The other physical children of Abraham are under the law and are lost.

Genesis 28:1Then Isaac called Jacob and blessed him and directed him, “You must not take a wife from the Canaanite women. 2 Arise, go to Paddan-aram to the house of Bethuel your mother's father, and take as your wife from there one of the daughters of Laban your mother's brother. 3 God Almighty bless you and make you fruitful and multiply you, that you may become a company of peoples. 4 May he give the blessing of Abraham to you and to your offspring with you, that you may take possession of the land of your sojournings that God gave to Abraham!”

I was interested in your comments on this passage:

Genesis 17 v.19 Then God said: "No, Sarah your wife shall bear you a son, and you shall call his name Isaac; I will establish My covenant with him [not Ishmael] for an everlasting covenant, and with his descendants after him. 20 And as for Ishmael, I have heard you. Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly. He shall beget twelve princes, and I will make him a great nation. 21 But My covenant I will establish with Isaac, [not Ishmael] whom Sarah shall bear to you at this set time next year."

God establishes an everlasting covenant with Isaac, Not with Ishmael, how can you explain this if they are both in the same position covenantally?

And I will repeat these questions since you didn't answer them the first time.
[*]Do you believe that Ishmael was in the Covenant of Grace?

[*]God shows grace only to His elect, so how could an unbeliever even be in that Covenant?

[*] And even more importantly, when you baptize an infant, do you say he or she is "in the New Covenant" and "in Christ"?