"Why would God be sorry that He made man if they were made and were acting in accordance with God's divine will?"
God has two wills. His plan include things that He permits but He does not like. He is sorry that it has to be like this but it must be for whatever His divine reasoning hold.
If this seems stupid I would remind you of the story of Elisha and Hazael (2 King 8) in which the Lord showed Elisha that Hazael would be king but Elisha knew the evil that Hazael would cause. God knew that Hazael would burn the villages, kill children and women, etc. God didn't tell Elisha to go and kill Hazael before he could cause this great evil. God let it happened. And because He is the one who sets up kings God made it happened. Do you think that God "repented" that He made Hazael king?
If God changes His mind then our salvation is suspect. What happens if tomorrow God changes His mind about that? What happens if God changes His mind about the mode of salvation?
Open theist is heresy of the worst sort abscribing man's attributes to God. To say otherwise contradicts God's own words in Numbers and 1 Samuel. Of course I have seen no interpretation for those verses. The early church fathers never once felt God "changed His mind" and I'm shocked, though not surprise, to see some of our Arminian friend say otherwise.
Anyone who entertains such a notion has a VERY poor concept of God and a poor understanding of God's word.
Do you think that most Calvinists would agree with that statement?
Other than Piper, is there any other renowned Calvinist that makes that assertion?