I have downloaded WIAonTWAIN in order to use it as a "vitual WIA device".
Unfortunately, it won't work as is.
The documentation should say that you need to install "TWAIN sample Data Source and Application" (the latest available is: twainds.win32.installer.2.1.3.msi).
Also I had to edit the file "TWAINDS_Sample32.ds" to replace the string "TWAIN2 FreeImage Software Scanner" with "TWAIN2 Software Scanner" (it is the product name).
I installed WIAonTWAIN_SDK.msi on clean Windows 10 32bit machine. No problems found. Scanned successfully with WFS and Paint. There is no need to install TWAIN sample DS. Please provide more details.