@Laurajm
You're very welcome!
I've not been down that road at the end of Mill Creek Road that branches off to the left and the right. It is marked as a "private road," with no foot travel allowed. (I know this from taking walks through Smithville, out to the athletic fields west of the park, and to that branch point.) It may be that folks were concerned about people coming around and scavenging around the damaged buildings and perhaps causing more mischief for the people still living there.
In addition to the possible loss by flooding of some of the houses, I seem to recall at least one or two more that were otherwise damaged. (A few years ago, there were pictures on one of the real estate company websites that appeared to show serious home damage at a location or two, at minimum. Checking one of these sites might give you a better idea of the possible condition of your grandparents' summer home.)
Here is a picture of the branch point from a few years ago. Do any of the signs, or the names on the signs, look familiar? I hope that you find good news, and enjoy your visit in November.