The monster advancement table for determining key classes says "* This class is only key if its spellcasting levels stack with those possessed by the creature." under Cleric, Druid, Sorcerer, and Rogue. Prestige classes aren't listed on that table, but neither are non-core classes, yet products like the Monster Codex seem to put the Cavalier in the same category as Fighter (a Fire Giant Cavalier 6 is CR 16). What about prestige classes that boost a class's spellcasting, like Mystic Theurge? Are Mystic Theurge levels counted as Key on, say, a red dragon (spell role monster), if they add to the monster's natural spellcasting?
What about prestige classes that don't add to spellcasting each level, like Eldritch Knight? If I put one level of Eldritch Knight on a spell role Guardian Naga, all that adds is +1 BAB and a bonus combat feat - clearly not key for a spell role monster. But levels 2-10 of that class will be adding to the Guardian Naga's natural spellcasting as if I were adding Sorcerer levels. What happens here?
I'm also not clear on this wording: "Classes marked with a “—” increase a creature's CR by 1 for every 2 class levels added until the number of levels added are equal to (or exceed) the creature's original CR, at which point they are treated as “key” levels (adding 1 to the creature's CR for each level added)." Is that levels added per class, or total class levels added? If I take an Ogre, combat role and CR 3, and put 3 Wizard levels on it, that gets it up to CR 4 1/2 (round up to 5). Then I start putting Rogue levels on it. Are those Rogue levels adding +1/2 CR each (since it has less than 3 Rogue levels) or +1 CR each (since the monster already has 3 class levels)?