Looks like you are on the right track in diagnosing the situation. I'm no expert by any means, so I may be wrong too, but your idea about the scanluns thing being redundant is reasonable, and may be correct.
Personally, I have had the rc.scanluns script disabled since installation, and have never used it.
If everything in your machine works and there are no problems or issues with any devices, then leave it disabled.
Thankfully. slackware and its scripts tend to be very well commented
which is often beneficial. As for what the differences are between your otherwise identical kernels, either check, check, check again for something you've missed, or consult the kernel release changelog.txt for indications of maybe a coding change affecting this.