Hmmm... so you think a person can't become corrupted, if he was chosen by God? Why do you think so?
And please give the scripture, where it is said Solomon became a pagan?
I think exalted souls paths are known by God when they pledged to him. He knew they would be truthful. I believe there is a category of exalted souls above great souls, and followers of exalted souls can't reach them.
I believe it's rational to believe God transcends exalted souls in exaltation, but that exalted souls are holy, pure, and guaranteed to remain righteous.
Islam is a middle ground between worshiping such souls or belittling them. The middle ground is we exalt them, and not fall short of that nor go ahead beyond it. This is the middle path.
I found this link:
Solomon drifted away from God because of his wives. It is questioned if Solomon died in a state of broken fellowship with God or if he came back to Christ.
www.neverthirsty.org
Now King Solomon loved many foreign women along with the daughter of Pharaoh: Moabite, Ammonite, Edomite, Sidonian, and Hittite women, from the nations concerning which the LORD had said to the sons of Israel, “You shall not associate with them, neither shall they associate with you, for they will surely turn your heart away after their gods.” Solomon held fast to these in love. And he had seven hundred wives, princesses, and three hundred concubines, and his wives turned his heart away. For it came about when Solomon was old, his wives turned his heart away after other gods; and his heart was not wholly devoted to the LORD his God, as the heart of David his father had been. 1 Kings 11:1-4 (NASB)
Now the LORD was angry with Solomon because his heart was turned away from the LORD, the God of Israel, who had appeared to him twice, and had commanded him concerning this thing, that he should not go after other gods; but he did not observe what the LORD had commanded. So the LORD said to Solomon, “Because you have done this, and you have not kept My covenant and My statutes, which I have commanded you, I will surely tear the kingdom from you, and will give it to your servant.” 1 Kings 11:9-11 (NASB)