Hang on, when was it stated that sacrifical slabs only can be used with one life left? I honestly can't remember seeing any mention of a ceretain amount of lives limiting the use of quest beds/cocoons or slabs.

It’s implied.  Every time a Quest Bed has been used the player had two lives, and every time a Sacrificial Slab has been used they’ve had one.  I guess it could just be a coincidence, but I doubt it.