You have run across an interesting idea insofar as the different size iron cross on various graves. Your future work could help others in trying to determine the date of the stone. It would appear that in different years the government may have had different sizes of the iron cross on the stone markers. What you're running across is raw research. With raw research you're pretty much on your own.
If I were you I would start by doing some stats. You must have a starting point and that point would be knowing for sure when a stone was placed on a grave that has an iron cross on the headstone. For example if you know from cemetery records that a Confederate headstone was placed on a grave say in 1935 and then measure the iron cross. Let's just say that it was 3 and 1/2 inches in diameter. Make a note about it. Then look for another known grave that has a marker and when it was placed. You might find that in 1938 that the iron cross on the headstone is 3 inches in diameter. By keeping records you might be able to date a stone using the information you have compiled. Your work has already had some great results. Now we know if a Confederate headstone had no iron cross that it probably dates pre 1930 if it was of government issue.
Your last four questions I have no idea on how to answer them. Perhaps the NCA in Washington D.C. might be able to answer some of them.