Ok, this is all fine with me.
I can get on board with Supernatural and random, but it should stick to Strength-based. Sure, the save DC is high, but critical hits aren't too difficult to divert at mid-levels (fortification armor immediately leaps to mind).
18-20/x3 is standard for augmented critical.
Training a Quickbiter...It'll cost you an arm and a leg.
10 improving to 15 sounds good.
Make Ethereal Plane its home plane (since we've linked them to ethereal marauders)?
True enough! Ok, very good.
Did we decide on ranks?
It's more the "giving its targets a -2 penalty to surprise rolls" bit in the room description.
I took this more as the "quick" part of its name, not any skills at ambushing. It seems to just be waiting there, and is so swift that it "gets the jump" on those entering the area. Hence Improved Initiative as a bonus feat.
So I propose Listen, Move Silently, and Spot, like the Ethereal Marauder, although I'm fine with adding Jump (or even Tumble).
That's the beauty (and problem) with these abbreviated statblock creatures that only appear in modules...much is left open to interpretation.
Again, I agree with Cleon (that's really remarkable how often this is happening these days)--they're stealthy and springy, what with them hiding behind columns and things. A racial bonus to Hide strikes me as appropriate.