If I take the new Mastery node that says "Elusive also grants +40% Critical Strike Multiplier for Skills Supported by Nightblade" and I then equip gear/passives that increased the "effect" of Elusive, does that also increase the 40% crit multi, since that is now "part of" Elusive buff? Assuming I'm using a skill supported by Nightblade.
Yes. Note that Elusive works by scaling it's effect down until it's removed at 0% effect, so you won't be getting the full bonus the whole time.