I try to hold it like the second pic but often find myself reverting something closer the first pic. Think I have better feel and it feels just as comfortable as second pic. Tend to agree that thumb down produces more topspin. Not sure why... Suspect many students use the first pic and find it comfortable and that is why Kevin made a video advising against it.FWIW, I have held my racquet like the first pic ever since after I broke my hand back in high school, and while my backhand is quite good, I have never seen a single other player (rec or pro) who uses my grip, except for maybe Don Budge from like 7000 years ago.
I use the thumb-out grip specifically for MORE stability, and I believe it definitely provides more stability. It does however severely limit your racquet head speed and ability to create spin.
Yes it's a subconscious thing that only happens sometimes when I'm particularly concerned with placement. It makes me feel (perhaps falsely) like I have more control. I actually notice it more on dropshots than aggressive slice.Have you tried allowing your thumb to naturally go around the racquet? The thumb around bevel 7 seems awkward for slice.