A widow is A SINGLE CHRISTIAN PERSON, so what Paul told her about seeking a mate would apply to ALL SINGLE CHRISTIAN PERSONS.
You are making a leap the text does not support. This is based only on your assertion. It is not unreasonable to think that marriage should function the way it did pre-Mosaic covenant (which continued under the Mosaic covenant.) I Corinthians 7 has a section on virgins getting married which speaks of giving a virgin in marriage. It does not say that the widow is given in marriage. She may marry whoever she wills.
The Bible does not divide people into 'single, divorced widows.' It has categories like virgin, defiled woman, widow, etc. (E.g. in the passage about priestly marriage regulations in the Old Testament. Priests could only marry virgins or widows of other priests.)
God gives us an amazing amount of freedom in our choices in life...as long as we do not sin against God in our endeavors.
I would agree with that.
In the case of Isaac and Rebecca, it was a unique situation and cannot really be used as something that we think will happen to us.
Jesus gave some great teachings on prayer, and Abraham and his servant probably had no idea of this. Jesus said all things are possible to him that believes. But, even so, they prayed, and the Lord directed who Isaac was to marry.
God may specifically direct a couple of they pray for it. My wife and I were both praying for direction. He can also direct people marry specific individuals if He so chooses, or direct any other area of an individual's life.
I remember a young Christian lady years past that stood before a lot of other young ladies and told us all that God had told her to marry her husband when he was not saved and was alcoholic. God did not tell her that because a Christian can only marry a Christian and marriage is the choice of the individual.
People need to be very careful with the "God wants me to" stuff. Its ironic to me to see people who don't believe God communicates outside of the Bible to ascribe motivations to God that He hasn't revealed in the Bible or elsewhere. There are people who will say that God did this or that, or that such and such was God's will because it happened. We shouldn't prophesy falsely, and we should not be false witnesses of God, either.