That it's not in itself a complete solution, is not an argument about the security value of this precaution. They can't escape on their own if their limbs are destroyed or (say) stored on another continent.

They could move a little even without their limbs.

What of it, they should be kept in cells in any case.

If kept conscious and intact, they could talk to each other and coordinate,

Or kept in solitary cells.

they're adopting a consistent strategy for the entire group.

"Consistency" (simplicity, rather) is a poor argument for picking a bad strategy, making a possible disaster worse. If only 5 extra-witchy vampires escape, it's better than if all 16 vampires escape. It also expends more resources of maintenance. As you wrote:

It's quite a production to handle the healing-and-feeding phase (Alec or Alec!Adelaide needs to be available, you need to fill up whoever feeds them so their food doesn't get stolen by inadequately fed guards, you need to have extra food on hand for the witches themselves, etc.).