Solved (partially): This makes no sense, but after struggling with it off and on for several days, this line began to work:
set WinDrive to disk "[C] Windows 10"
At first much of the Windows disk was available, but "Program Files," "Users," and other parts were not. In "Windows 10" Configuration "Access Windows folders from Mac" was checked, but under Advanced Settings, "Share Windows-connected drives with Mac" was not. Once I checked that advanced setting and restarted Windows, everything started to work as they had before updating to Mojave.
Edit: 15 minutes after posting this, while running a script that copied files from the Windows drive, the Windows drive suddenly went offline. I was able to go back into Windows via Parallels and manually copy files from Windows to the corresponding Mac directory.
Last edited: Mar 24, 2019