Yes, the Battle Arms Development Pistol Length Lightweight Stock Combo / Qd End Plate can be used with an AR9.
Yes. The Battle Arms Development Pistol Length Lightweight Stock is compatible with GearHead Works Tailhook MOD 1 with Stepped down OD (outside diameter) at end of the buffer tube.
Length Past Lower Receiver = 6 5/8in
No this stock isn't adjustable.
This is the pistol length tube, which is not made to work with the stock plate the goes on the end. Only their mid and rifle length tubes have the mounting area for that stock plate.
The diameter of the dimpled buffer tube is 1.25".