I don't advocating cutting off the thumb, but don't use it to grip. That is to say, apply no pressure with the thumb. It's the fingertips that share the weight, and give direction. As stated, I think the best allemande hold uses the thumb, but not to grip. It's important to be able to separate yourself from your current partner at your discretion.
Thumb pressure is what caused the current trend in yucky allemandes.
Let's work on that problem as we bring back the original hold. The original allemande With the original allemande there should be no grip. The thumbs cross, but do not bear down. The fingers curve around our allemande partner's fingers, but do not grip. This hook shape we make with our fingers hooks into the other dancer's hand.