The Drakainia has an ability called Razor Claws, which gives her claws a threat range of 17-20/x4. But the statblock gives her claws as a range of 16-20.
So which one is right? The ability or the statblock?
Possibly just a misprint. I would use the ability (17-20/×4) as is seems to be copying the rules for Improved Critical (Treating claw attack as a 19-20 range doubled). 16-20 is a weird range as 15-20 or even 17-20 is the standard for improved crit ranges.
I don't think a 16-20 threat range is even possible in Pathfinder.
So if she gained Improved Critical for the claw, the claw would threaten on 13-20, right? Or would it not stack any further?
But it’s just one enhancement. The 17-20 is the starting threat range for the claw, like how a greatsword starts with 19-20 and can get 17-20 through Keen or Improved Critical, but not both.
Claws which have a crit range of 17-20/x4 are definitely under the effect of something, and 'This effect doesn't stack with any other effect that expands the threat range of a weapon.'.
Would Mythic Improved Critical still up the crit multiplier to x5, even if it doesn’t alter the threat range?
Mythic improved critical doesn't have any awkward stacking rule so that's fine.
I don't think a 16-20 threat range is even possible in Pathfinder.
If you let a inspired blade use something other than a rapier, say a longsword, their +1 threat range would make their improved critical threat range be 17-20+1=16-20. But its the only ability I know that increases threat range by 1 and it requires a rapier.
Except Inspired Blade abilties only work with rapiers.
The only true way to get a 16-20 range is through homebrewing abilities to increase the critical range of an improved range of 17-20 by 1, as AwesomenessDog was portraying.