All cabins can be booked specifically if available. Call
DCL to see if available.
Often cabins don't show up available online for various reasons.
You do not need to choose the GTY option to get this cabin. In fact, that the last path you want to take to get one of the aft wrap around cabins.
You have to book early, like when the dates are released, to have a good chance at booking one of these category 5e corner wrap arounds.
These are popular cabins and book up quickly once dates are released.
9670 is not a handicap cabin, but 9170 is, fwiw.
Decks 7 and 8 also have these non HA monster corner aft cabins too fwiw.
Some have experienced vibration in the aft. It seems to vary though, sailing to sailing. I believe it depends on specific characteristics such as speed, waves, wind, and current. Some people are also more sensitive to said vibrations. It's difficult to predict how one may be affected by aft vibrations. The vibrations seem to vary widely according to reports too.
What ship are you considering?
HTH