actually, it wasn't until you brought up petrify as a term that "stone" in any context came up. was looking for a "im so scared im going to wet the bed if i dream of this tonight" term. petrify does have a component of fear, but is also very commonly used, as in this very game, to denote being turned to stone. and while lapidify is a perfectly usable term for such, it isnt a well known word, so uncommon that it comes up as a misspelled word on firefox's spellchecker, and on thesaurus.com it ironically sends you to its 'petrify' web page.
OT- im not sure what you mean, the ability is static, the damage done and health drained by nightmare is the same for both cards and entirely dependent on how many copies are placed into the opponents hand.