To answer my own question: I decided to back up the file containing the passwords and try it out. My worst fears were realized, and the passwords were indeed deleted from the unsynced device.
I can get back into my starting position by copying the file I'd backed up back into its original location and restarting Chrome. This time, because sync was turned on, ...
One possible solution is
Click on the 3 vertical dots in the upper right corner
Click "More Tools" > "Clear Browsing Data"
On the new page, change the time frame to "All Time"
Select "Passwords" before clicking on "Clear Data"
There is no way to restore passwords after doing ...