Your observations are the same as mine other than I've not experimented with switching WDFME off, but irrespective of that, most folks don't do that, they shut down their PC which allows files to accumulate.
WD was very responsive to my postings on this issue. Initially I was told that the retention of the sqlite files in the Windows temp folder was a MS Windows problem, and the WD software was working as it should. Subsequent to that I was told that the next release of WDSmartware will continue to place these files in the temp folder, but that they will be significantly smaller than the 2GB per day files users are now getting.
In the mean time, I suppose we all have to manually erase the temp folder and empty our recycle folder daily. That's what I do, and now it has become habit.
I have a 128GB solid state drive as drive 0 on my PC, and I strive to keep about 1/3 of it free, so this is a real issue for me as well.
I wanted to clarify something that you mention in your post. WD is going to address this issue, however, these files will still exist. We are reducing the amount of information stored in these files as apart of other back-end changes related to CPU and memory usage. These files will be significantly smaller.
What you are seeing here isn't a "bug" per se as the software is working as designed, but this is a bad user experience which will be addressed.
Hope that clears things up.