Glad you found the old nickels and the wheat cent. And you are correct in your statement as to why many others may have missed them. Many other detectors, and most built prior to the past few years, incorporate variable discrimination. That means once you set the discrimination, it will reject every target with a higher ferrous content. But I disagree with your comment that the nickels bounced around from 8 to 22 on the TID. I would be willing to bet that the bouncing is due to the adjacent nails and not the nickels. When you sweep a coil over multiple targets, the TID will attempt to provide an indication for each target. The closer the targets or the faster you sweep, the more quickly the TID has to change. If you were to first remove the nails, I think you would have found the nickels to provide a consistent reading. Individual US coins will typically read the same in the dirt as they do in the air. The exceptions being when the coin is adjacent to another metallic object(s), if the coin is on edge, the detector is improperly setup for the site or if they are too deep to provide accurate phase shift information.
Your choice of using the 6-inch DD coil was the proper choice for this type of hunt. Larger coils or concentric coils will not provide the separation you got with your 6-inch DD. Congrats on the finds and for selecting the right tool for the job! HH Randy