I don't see why it wouldn't be. I owned one and can say that the difference between the AF in the A9 and the A1/A1ii is not as big a difference as it is between the A9 and A7Riv.
As long as you get it for a good price, yes. Don't over-pay for it.
It had an unusual lock on the lower left dial - a button separate from the dial. A friend of mine dropped hers on that corner and the lock wouldn't operate. I prefer the lock on the mark II, or later (A1 series, A9 III). Just something to be wary of.
If you mean focus stacking, the answer is never - focus bracketing requires a more powerful processor - the A9 and A9 II have the much older processor.