I had the option of surgery to treat the joint at base of my left thumb which "popped" out of place a few years ago. A sort of "knot" formed at the base of my thumb. For a while there was some pain involved and my pinch power with left thumb and index finger was weakened. Eventually the pain subsided and I can easily get along with weakened pinch power, so I chose to do nothing about it. It's still there, but I don't think it has much, if any, effect on my golf grip.
Re my allowing a slippage of trail-hand fingers around the shaft during takeaway. I should say that there's nothing in GTTB to suggest that Mindy employed or recommended my adaptation. He described the grip change action as trail hand bending back early in takeaway and the two hands coming together naturally during the takeaway.
You didn't answer my question re whether you attempt to move your hips forward during takeaway/backswing. Jim