Good questions.
Church is steady but is limited due to a lack of speed. They have to be careful, because they could end up with a worse player if they move on from Church and depend on a young guy. On the other hand, it would be helpful to have a Strong Safety with more range. Ideally you would like to be able to have the Strong Safety occasionally cover the TE in man coverage, but Church can't do that. The Cowboys have some LBs that are likely faster than Church. Another thing to consider is that as speed limited players age, they don't have much margin before they are just too slow. The super fast guys can lose some speed as they age and often still be OK.
Wilcox has played quite a bit even when Church was not out. When they play the 3-2 package, that has 6 DBs on the field. The Dime defense is a 4-1-6 which again has 6 DBs on the field. I have not really studied Wilcox much this season but from what I have seen he has been much better than in past years. It's odd because he was his absolute worst ever this preseason. I think in retrospect, that the new DB coaching hierarchy must have been forcing him to learn to play 'their" way and to unlearn what the previous DB coach had taught him. The Cowboy would be OK if he replaced Church but he would likely be a player that you would want to upgrade if possible. His role this season as a 5th or 6th DB was ideal for him, IMO.
Carr is a difficult decision and it obviously depends on the cost. He has never missed a game or start in the NFL. There is value in that even if he is just a mediocre player. He is another speed and athletically limited player and he is 30 which means that he will just get slower as the months pass. Also Carr has said he is considering retirement.
Claiborne is another difficult decision. He played really well in the first 7 games before being injured. He has never played a full season and has only played about 3 of 16 possible preseason games. Even at his very best, he is not a ballhawk (He was in college but not in the NFL). I expect them to sign him to another 1-year, 3M deal.
Scandrick is showing signs of decline and might be approaching injury prone. He is under contract, but these issues just add to the need for more talent to be added at the CB position. Scandrick has the worst hands that I can remember seeing for a starting NFL player.
Brown was terrific considering that he was a rookie. Some people will complain about a few bad plays, but he played well over 700 snaps as a rookie and only had a handful of bad plays.
Leon McFadden is the last CB on the roster. They didn't even have a CB on the practice squad. He did OK in very limited action. I can't really blame him for giving up a TD to possibly the best WR in the league on a trick play. He is smallish and not overly fast and this is his 3rd season of hanging around the NFL so his upside is limited.
Summary: They could stand to both sign a quality free agent CB (maybe Trumaine Johnson) and to use a premium pick on one in the draft. I doubt if they use a premium pick on a Safety but if the right opportunity comes along, I would do it.