For 1), I feel like the current questions are reasonable guidelines, though the language could use some reworking. For example, a +2 availability Catch shouldn't just be something that anyone can get their hands on; it should be something that anyone can easily acquire in a dangerous form. Cold iron certainly counts, as any home, workplace, or well-equipped car trunk has at least one mostly-iron item with the weight and heft to be used as a bludgeon or the pokiness to be used as a shiv. Fire, likewise, is inherently damaging, and can readily be generated. But something which is hard to find, even if it doesn't take True Magic or some other genuinely-limited resource to employ, shouldn't get that same benefit; anyone could theoretically acquire a bottle of holy water, but it's not common. Likewise, things that are hard to weaponize either shouldn't count, or should render the creature vulnerable to all attacks made after exposure; if garlic is your only weakness, you should either burst into flame when exposed or have your hide suddenly rendered soft and delicate. It shouldn't take actual garlic-induced blunt trauma to do you in.
Again, for commonality of knowledge, I'd make +2 something that either anyone who knows anything at all will know, or something that will almost certainly be arrived at by trial and error in the course of a given combat. The classic superhero Toughness, which doesn't apply to actual penetrating injury but shields against ludicrous amounts of blunt trauma, would count here; likewise silver against werewolves, which almost anyone who thinks they know what a werewolf is will try.
As far as point 2) goes, I think it's quite reasonable to simply design a +0 Catch as follows: Only by weapons which bypass all defenses, requires direct knowledge.
