I'm not making assumptions.
Of all things models, textures, animations and particles are in fact that easy to change, even for people not associated with Riot.
Not to mention I'm sure Rengar's model haven't changed, because as I said before it'd be fool's errand unless they have technology in place to make modifications to champion rigs that easy to apply.
And I'm 110% sure they don't.
Remember how Rioters were talking about the making of Super Galaxy Rumble skin? They had to in fact create that skin completely from grounds up, because his "classic" rig is unusable in it's current state and yet it's still here.
Any change to a champion's rig multiplies the amount of work needed, that's why any logical person isn't going to change it, unless absolutely necessary.
Yes, swapping is easy, creating a new entry for a new crit attack could prove harder.
I don't disagree with that.
As for Mordekaiser glitches. Everybody knows his rework was half-assed, so I'm not surprised he has many bugs.