Actually, according to scripture, it seems to indicate God is the author of sickness (and all other forms of judgment toward sin):
Here are some examples from scripture to support my statement (although it is definitely not an exhaustive search):
11 Then the Lord said to him, "Who has made man's mouth? Who makes him mute, or deaf, or seeing, or blind? Is it not I, the Lord? (Exodus 4:11)
then I will do this to you: I will visit you with panic, with wasting disease and fever that consume the eyes and make the heart ache. And you shall sow your seed in vain, for your enemies shall eat it. (Leviticus 26:16)
While the meat was yet between their teeth, before it was consumed, the anger of the Lord was kindled against the people, and the Lord struck down the people with a very great plague. (Numbers 11:33)
5 But to Hannah he gave a double portion, because he loved her, though the Lord had closed her womb. 6 And her rival used to provoke her grievously to irritate her, because the Lord had closed her womb. (1 Samuel 1:5-6)
And about ten days later the Lord struck Nabal, and he died. (1 Samuel 25:38)
And the anger of the Lord was kindled against Uzzah, and God struck him down there because of his error, and he died there beside the ark of God. (2 Samuel 6:7)
And when the Syrians came down against him, Elisha prayed to the Lord and said, "Please strike this people with blindness.” So he struck them with blindness in accordance with the prayer of Elisha. (2 Kings 6:18)
And that night the angel of the Lord went out and struck down 185,000 in the camp of the Assyrians. And when people arose early in the morning, behold, these were all dead bodies. (2 Kings 19:35)
And after all this the Lord struck him in his bowels with an incurable disease. (2 Chronicles 21:18)
And Azariah the chief priest and all the priests looked at him, and behold, he was leprous in his forehead! And they rushed him out quickly, and he himself hurried to go out, because the Lord had struck him. (2 Chronicles 26:20)
The ten plagues of Egypt
Moses and Aaron did as the Lord commanded. In the sight of Pharaoh and in the sight of his servants he lifted up the staff and struck the water in the Nile, and all the water in the Nile turned into blood. (Exodus 7:20)
you shall say, ‘It is the sacrifice of the Lord's Passover, for he passed over the houses of the people of Israel in Egypt, when he struck the Egyptians but spared our houses.’” And the people bowed their heads and worshiped. (Exodus 12:17)
for all the firstborn are mine. On the day that I struck down all the firstborn in the land of Egypt, I consecrated for my own all the firstborn in Israel, both of man and of beast. They shall be mine: I am the Lord.” (Numbers 3:13)
Examples from the new testament:
5 When Ananias heard these words, he fell down and breathed his last. And great fear came upon all who heard of it. 6 The young men rose and wrapped him up and carried him out and buried him. (Acts 5:5-6)
And now, behold, the hand of the Lord is upon you, and you will be blind and unable to see the sun for a time.” Immediately mist and darkness fell upon him, and he went about seeking people to lead him by the hand. (Acts 13:11)
27 Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty concerning the body and blood of the Lord. 28 Let a person examine himself, then, and so eat of the bread and drink of the cup. 29 For anyone who eats and drinks without discerning the body eats and drinks judgment on himself. 30 That is why many of you are weak and ill, and some have died. 31 But if we judged ourselves truly, we would not be judged. 32 But when we are judged by the Lord, we are disciplined so that we may not be condemned along with the world. (1 Corinthians 11:27-32)
Not to mention the trumpet and bowl judgments of the book of Revelation where it is clearly that God is the One who is destroying the world and all who dwell on it.
Now, I will try to address some of the other points you brought up by breaking them down into small sections.
In regards to your statement: "Is is God's will for His children to be under bondage to the enemy, when Jesus has given us the victory?"
Not quite sure what you mean by bondage, but it is clear that Paul had a sort of "bondage" or a restricting influence over him in the form of the thorn in his flesh. It is also clear from Revelation that God actually will give the enemy the authority to prevail against the saints (in regards to killing them):
Also it was allowed to make war on the saints and to conquer them. And authority was given it over every tribe and people and language and nation, (Revelation 13:7)
Clearly, the "victory" over the enemy is not something fully actualized until the end is complete, and that they enemy is very much allowed, rather used by God for His purpose, even for our good.
Regarding your statement: "Is it God's will for His children to be sick when He not only forgives us for all of our sins, but He heals us from all of our diseases?"
This is what I was talking about in my previous comment, that the healing purchased by the atonement is not to be fully actualized until the day of Judgment and our glorification. And yes, it is many times the will of God for us to be sick, just as much as it is the will of God for us to undergo much hardship and suffering in this life, and even to the extent of the enemy "conquering" the church for a time, because this body counts for nothing, it is a tent and not my real home.
Regarding your statement: "Just as sin is cleansed by faith in the blood, healing comes through appropriating the finished work of Jesus Christ."
I absolutely agree, that whatever healing one receives ultimately comes back to the atoning work of Christ on the cross but what I am saying is that it doesn't give me a sure-found guarantee that I will receive healing or that my receiving of healing in the here and now is always 100% the will of God because His purpose for us on this side of eternity is not prosperity but rather sanctification.
Regarding your statement: "Are we sure that Paul's thorn in the flesh was sickness and disease? "
Although the implication is strongly toward a bodily ailment, it is a moot point to argue because if sickness is outside the will of God and therefore it is something caused by the devil, we find clear precedence from the thorn in the flesh example that God even uses the devil himself for the good His children.
Now just to be clear, my heart is FOR healing and I believe with all my heart that the Lord heals TODAY, but I cannot accept the teaching that every sickness is outside the will of God because the bible seems pretty clear in my opinion that it is God who gives and takes as He so chooses and that He uses everything, whether sickness or the devil himself for His sovereign purposes. Now one might argue, how can we really believe God for a healing then, it might be His will not to heal me. But I would then say, let us be careful not to presume to know the mind of God and put ourselves in God's shoes and say who He will heal and who He will not heal, but rather come to Him knowing that He is our faithful and compassionate Father who hears our requests and will answer accordingly in His boundless wisdom and love toward us whatever would be the best means to make us more like Christ, which in the end is what really matters.