I actually have made contact with your Sis-in-Law Carol. She and I and another 'cousin' have been emailing for the last couple of years and sharing things, including pictures from the cemetery. I've made a couple of trips down there, and you are right - the marker is gone from Rudolph's grave. I believe Carol was working on a replacement before they moved, and now that my DAR membership has gone through, perhaps I can work on it from my end.
I would be interested in connecting with Isaac's descendents, as that's my line.