I can backup my SQL Server 2008 R2 but can't restore from the very same folder
found it. turns out the permissions command I was using isn't enough for a restore (it's enough for the backup).  Granting the full access rule on the folder isn't enough - it has to be on "the folder, subfolders and files".

basically two more lines of code:

myDirectorySecurity.AddAccessRule(new FileSystemAccessRule(sqlserviceuser, FileSystemRights.FullControl, InheritanceFlags.ContainerInherit, PropagationFlags.None, AccessControlType.Allow));

myDirectorySecurity.AddAccessRule(new FileSystemAccessRule(sqlserviceuser, FileSystemRights.FullControl, InheritanceFlags.ObjectInherit, PropagationFlags.None, AccessControlType.Allow));