David was fallen man, but he (sometimes) chose to do God's will. All the prophets were fallen but obeyed God.
The regenerate man is "able not to sin (posse non peccare)", which also means that he is able to sin. Would you argue that that David and the prophets were not regenerate?
(As a side note, I would point out that outwardly following God does not necessarily mean that one is trusting in God for salvation, and that is why some may appear to be outwardly Christian, but their hearts are far from God. That is why works are an indicator of one's salvation, but only an indicator.)
Abram had not yet been redeemed but did God's will.
What indication do you have that he was not redeemed? Was not Abram chosen by God? Did Abram chose to do this before God's call?
Well, I'm not sure how it worked with Abram. As you say, he was called by God and God tested him with Isaac (and in many other ways).
But as said, I'm just not sure how redemption works for the elders (since Jesus had not yet been to the Cross).
I was hoping you could tell me.;-)